Russia on Wednesday said it was expelling US diplomats in retaliation for Washington's step to remove 12 of Moscow's representatives to the UN based in New York.

"On March 23, a note with the list of the American diplomats declared 'persona non grata' was handed to the head of the American diplomatic mission who was summoned to the Ministry of Foreign Affairs," the ministry said in a statement.

The decision was taken in response to the expulsion by Washington of Russian diplomats at the UN in New York, the source said.

"The US side has been given firm notice that any hostile action by the United States against Russia will be met with a resolute and appropriate response," the statement added. 

A US State Department spokesperson confirmed it had received the list from Moscow.

"This is Russia's latest unhelpful and unproductive step in our bilateral relationship. We call on the Russia government to end its unjustified expulsions of US diplomats and staff," this official said.

"Now more than ever, it is critical that our countries have the necessary diplomatic personnel in place to facilitate communication between our governments."

The United States expelled 12 members of the Russian diplomatic mission to the UN in early March amid an open crisis with Russia over Ukraine on charges of espionage. 

"The US has informed the Russian Mission that we are beginning the process of expelling 12 intelligence operatives from the Russian Mission who have abused their privileges of residency in the US by engaging in espionage activities that are adverse to our national security," the spokeswoman for the US mission to the UN, Olivia Dalton, announced in a statement. 

The Russian ambassador to the United States, Anatoli Antonov, denounced it as a "hostile move" by the United States, stressing that the actions were a cause of "deep disappointment and absolute rejection" in Moscow.

Ukraine calls for worldwide protests against war

Ukrainian President Volodymyr Zelensky made an impassioned plea Thursday for citizens worldwide to pour onto streets and squares in global protest against Russia's bloody month-old invasion.

In a late-night television address from the emptied streets of his nation's besieged capital Kyiv, a defiant but visibly tired Zelensky appealed in English for worldwide solidarity.

"The world must stop the war," he said. "Come from your offices, your homes, your schools and universities, come in the name of peace, come with Ukrainian symbols to support Ukraine, to support freedom, to support life."

His appeal came one month after Russian tanks rolled over the border, bringing a conflict that has killed untold thousands of civilians and soldiers on both sides. 

More than ten million Ukrainians have already fled homes and cities under sustained Russian bombardment from land, sea and air.

There is growing evidence that Russia's once-vaunted military has become badly bogged down, and has been forced to turn to long-range bombardment to break Ukrainian resolve.

In the southern port city of Mariupol alone, 100,000 people are trapped without food, water or power and enduring fierce shelling by Russian forces.

In the city's hospital, local officials said staff have evacuated patients to the basement, where they are treated by candlelight beside 600-700 other local residents seeking what little safety they can.

The US government on Wednesday said the Kremlin's bombing campaign amounted to war crimes, further escalating a confrontation between Moscow and the West that has rivalled the worst crises of the Cold War.

"We've seen numerous credible reports of indiscriminate attacks and attacks deliberately targeting civilians, as well as other atrocities," said Secretary of State Antony Blinken. 

"Based on information currently available, the US government assesses that members of Russia's forces have committed war crimes in Ukraine."

So far the conflict has not spilt over into direct military conflagration between Russia and NATO, but there are growing fears Russia may up the ante with a chemical, biological or even tactical nuclear attack.

US President Joe Biden is in Brussels for back-to-back emergency NATO, G7, and European Union summits from Thursday that will bring pledges of more lethal weapons for Ukraine, more punishing sanctions on Russia's already crisis-wracked economy and warnings about further escalation.

- Ukrainian resistance -

NATO officials believe that -- armed with an arsenal of Western anti-tank and anti-aircraft weapons -- Ukrainian forces may have already killed as many as 15,000 Russian soldiers and wounded perhaps 30,000-40,000.

Putin's regime officially puts the number of Russian fallen at under 500, and has introduced draconian censorship laws to prevent independently verified news about what it calls a "special military operation."

But Ukrainian civilians continue to bear the brunt of the war.

Zelensky admitted the last month had been "long" but hailed Ukrainian resistance that has been much more ferocious than Russia expected and would endure for as long as it takes.

"This is a war for independence and we must win," he said in the late-night address, flitting between Ukrainian and his native Russian. 

"We will rebuild every city, we will bring the invaders to justice for every crime," he said. "All our people will live in a free Ukraine."

Recent days have brought claims of Ukrainian forces not only repelling attacks from the much larger and much better armed Russian military, but launching counteroffensives and winning back territory around Kyiv.

Kyiv mayor Vitali Klitschko said that "the small city of Makariv and almost all of Irpin is already under the control of Ukrainian soldiers".

AFP journalists reported fierce exchanges of artillery fire in Irpin.

British military intelligence said Ukraine had "probably retaken Makariv and Moschun" to the northwest of the capital and "there is a realistic possibility that Ukrainian forces are now able to encircle Russian units in Bucha and Irpin."

- Putin responds -

Facing mounting diplomatic and economic pressure, Putin's regime has responded by warning Russia could use nuclear weapons if it faces an "existential threat", and launched tit-for-tat retaliation against the West.

Moscow moved to expel US diplomats in retaliation for Washington's move earlier this month to remove 12 of Moscow's US-based representatives to the United Nations.

In an effort to blunt the damage done by sanctions to the national currency, Putin said Wednesday that Russia will only accept payments in rubles for gas deliveries to "unfriendly countries", which include all EU members.

The manoeuver sharpened growing debate in Europe -- which is heavily dependent on Russian energy imports -- about possible bans on Russian oil and gas.

Moscow has warned an embargo would prompt a "collapse" of the global energy market. 

While Europe has appeared fractured on the question of an oil embargo, there are also signs of fissures within Putin's regime.

Moscow recently confirmed that Anatoly Chubais -- a former Kremlin chief of staff who oversaw liberal economic reforms in the 1990s -- quit his post as a Putin advisor. He has reportedly fled the country in protest at the war.

Russia still has a vital friend in China, which pushed back against suggestions that Moscow should be expelled from the G20 group of countries. 

But Scott Morrison, prime minister of G20 member Australia, said he believed that Putin attending the summit would be "a step too far."

- NATO reinforces -

In the latest sign that the war has reinvigorated NATO, alliance chief Jens Stoltenberg announced it would send more reinforcements to guard against Russia expanding its campaign.

The leaders at Thursday's summit will agree to "major increases of forces" including four new battle groups in Bulgaria, Hungary, Romania and Slovakia.

The allies will also offer "additional support" to Kyiv against nuclear and chemical threats, he said.

But NATO members, while maintaining a steady supply of anti-tank and short-range anti-aircraft missiles, have refused Zelensky's demands for a no-fly zone over Ukraine or shipment of warplanes to the Ukrainian air force, fearing all-out war with nuclear-armed Russia.

Nearly a month into the invasion, peace talks have agreed on daily humanitarian corridors for refugees, and Ukraine says it is willing to countenance some Russian demands subject to a national referendum.

But it has refused to bow to demands to disarm and renounce its pro-Western ambitions. 

Ukraine's lead negotiator Mykhaylo Podolyak said the peace talks were encountering "significant difficulties." Moscow accused the United States of undermining the process.

-  'Tip of an iceberg' -

The World Health Organization warned on Wednesday that the humanitarian disaster in Ukraine was only getting worse. 

"The problems we face so far... are really the tip of an iceberg of need," WHO emergencies director Michael Ryan said.

"And there's going to have to be a further, massive scaling up of assistance within Ukraine in the coming weeks, because I have never, myself, seen such complex needs, and so quickly in a crisis that has developed so fast."

"We have reached maybe, for once, an appropriate level of horror at what's happening in Ukraine," he said.

Latest developments

Here are the latest developments in Russia's war on Ukraine:

• Pentagon says Russia taking 'defensive positions' -

The Russian army has retreated more than 30 kilometres (18 miles) east of Kyiv and has begun to establish defensive positions on several fronts in Ukraine, a senior Pentagon official says. 

To the northwest, "they're basically digging in and they are establishing defensive positions," the official adds. "So it's not that they're not advancing. They're actually not trying to advance right now."

- More sanctions as Biden arrives in Europe -

As US President Joe Biden arrives in Brussels, warning of a "real threat" Russia may use chemical weapons, a top US official says new sanctions are coming against Russian "political figures" and wealthy elites close to President Vladimir Putin.

"We, the United States, will announce a package of sanctions designations tomorrow that relate both to political figures, (and) oligarchs... as well as entities," National Security Advisor Jake Sullivan tells reporters aboard Air Force One as Biden heads for NATO and EU summits on Ukraine.

- 100,000 trapped in Mariupol -

Zelensky says almost 100,000 people are still trapped in the ruins of Mariupol, after more than 7,000 escaped on Tuesday.

Human Rights Watch describes the southern port city as a "freezing hellscape riddled with dead bodies and destroyed buildings".

- US says Russia committed war crimes -

The United States has reached the assessment that Russian military forces have committed war crimes in Ukraine, US Secretary of State Antony Blinken announces. 

- NATO responds -

NATO chief Jens Stoltenberg tells an emergency Brussels summit allies will sign off on sending four "battle groups" to eastern members Bulgaria, Hungary, Romania and Slovakia.

He also signals cybersecurity assistance as well as equipment "to help Ukraine protect against chemical, biological, radiological and nuclear threats". 

- More German arms aid -

Germany says it is sending 2,000 additional anti-tank weapons to Ukraine to help it repel the Russian invasion, a parliamentary source tells AFP, confirming media reports. 

The Ukrainian forces have already received 1,000 anti-tank weapons and 500 Stinger-type surface-to-air missile launchers from the Bundeswehr, the German army.

- UK to send 6,000 missiles, money to Ukraine army -

Prime Minister Boris Johnson says Britain will send 6,000 missiles and £25 million ($33 million, 30 million euros) in financial aid to Ukraine's army to help it fight Russian forces.

The announcement of the funding and additional military hardware, consisting of anti-tank and high explosive weapons, comes on the eve of NATO and G7 summits set to discuss the Russian invasion.

- Renault suspends work at Moscow factory -

French car giant Renault announces it is immediately suspending operations at its Moscow factory after Kyiv calls for a boycott of the company for remaining in Russia.

Renault is also considering "the possible options" for its Russian affiliate AvtoVAZ, the company says in a statement.

- Russian journalist killed by Russian shelling -

A Russian journalist for investigative news outlet The Insider is killed when Russian troops shell a residential neighbourhood in the Ukrainian capital, the outlet says, the latest reporter to die in the war.

- 'Putin's offensive stuck': Scholz - 

German Chancellor Olaf Scholz says "Putin's offensive is stuck" and urges Moscow to "immediately" stop the fighting, saying it is not only destroying Ukraine but "Russia's future".

The Pentagon says it believes as much as 10 percent of Russian forces committed to Ukraine may have been knocked out in just four weeks of fighting and that Russian forces "have struggled with logistics and sustainment".

- Belarus expels Ukrainian diplomats -

Russia's ally Belarus says it is to expel most Ukrainian diplomats, accusing Kyiv of "interference" in its domestic affairs, leaving just five people from a 20-strong team at its embassy in Minsk. 

- Poland expels Russian diplomats - 

Poland says it has expelled 45 Russian diplomats for espionage, an allegation dismissed as baseless by Russia's ambassador to Poland. 

- Aid for war-hit European firms -

The EU gives member states the nod to offer limited subsidies and cheap loans to companies impacted by the war in Ukraine or sanctions imposed on Russia.

- Over 3.6 million flee -

More than 3.6 million Ukrainians have now fled the country following Russia's invasion, the United Nations says. More than 10 million have been displaced from their homes.