The United States was expected to announce tough new sanctions on Russia Wednesday, including a ban on new investments, a day after Ukraine's president showed the UN Security Council harrowing images of violence and accused Moscow of widespread atrocities.

The sanctions come after an outcry over the discovery of dozens of bodies in civilian clothing in areas from which Russian troops have withdrawn around Ukraine's capital, including the town of Bucha. 

Ukrainian leader Volodymyr Zelensky likened Russia's actions to Nazi atrocities in an impassioned speech by videolink to the 15-member UN Security Council on Tuesday. 

"They cut off limbs, slashed their throats, women were raped and killed in front of their children," Zelensky said. 

He demanded stronger action from Western powers and called for Russia's exclusion from the Council, where it holds veto power.

Later in his nightly address, a frustrated Zelensky said Russia was blocking the UN from carrying out "the functions for which it was created". 

"The UN Security Council exists, and security in the world doesn't," he said. 

In an interview with the BBC, the US ambassador to the UN admitted that "no one can question (Zelensky's) frustration with the Council and how the Council operates". 

But she insisted that the Council was holding Russia to account, and that Moscow was isolated within it. 

- Push to isolate Moscow -
The killings in Bucha and elsewhere have galvanised support for Ukraine, with Washington announcing another $100 million in military aid, and produced new momentum for additional sanctions on Moscow.

"We had already concluded that Russia committed war crimes in Ukraine, and the information from Bucha appears to show further evidence of war crimes," a US source familiar with the planned measures said. 

On Wednesday, Washington, in coordination with the G7 and the European Union, is expected to announce measures including a ban on all new investments in Russia.  

"You can expect... that they will target Russian government officials, their family members, Russian-owned financial institutions, also state-owned enterprises," White House Press Secretary Jen Psaki told a briefing on Tuesday. 

A new sanctions package being prepared by Europe, meanwhile, is set to include oil and coal, France's foreign minister said at EU talks in Luxembourg.

And Britain said it has so far frozen some $350 billion in assets from President Vladimir Putin's "war chest".

As part of the push to isolate Moscow, Spain, Italy, Denmark and Slovenia expelled dozens of Russian diplomats suspected of being intelligence operatives on Tuesday, following similar moves in France and Germany -- a total of some 180 expulsions in 48 hours.

The Kremlin called it a "short-sighted move" that would complicate efforts to negotiate an end to the hostilities.

Putin also warned of "reprisals" for recent European measures targeting Russian gas giant Gazprom -- and said Moscow would "monitor" its food exports to "hostile" nations, raising the spectre of shortages and price spikes.

- Kremlin denials -
The Kremlin has denied any civilian killings, claiming the images emerging from Bucha and other sites are fakes produced by Ukrainian forces, or that the deaths occurred after Russian soldiers pulled out.

At the Security Council meeting, Moscow's ambassador rejected Zelensky's claims, saying the "ungrounded accusations... are not confirmed by any eyewitnesses".

But satellite photos taken while Bucha was still under Moscow's control show what appear to be bodies lying in streets where the dead were later found by Ukrainian forces and seen by journalists. 

And multiple Bucha residents told AFP they had seen Russian soldiers killing civilians.

"Right in front of my eyes, they fired on a man who was going to get food at the supermarket," said 43-year-old Olena, who declined to give her family name.

During a grim cleanup, the remains of partially burned bodies in black bags were lifted into a van, with officials telling journalists "dozens of bodies" remained in apartments and in nearby woods.

Western nations have given short shrift to Russia's denials.

"What we've seen in Bucha is not the random act of a rogue unit. It's a deliberate campaign to kill, to torture, to rape, to commit atrocities," US Secretary of State Antony Blinken said.

European Commission chief Ursula von der Leyen, who will visit Kyiv this week, has offered the bloc's assistance in documenting proof of war crimes.

- Cluster bombs -
Ukrainian authorities have warned that others areas may have suffered even worse fates than Bucha. 

Scenes of devastation have met those venturing into areas from which Russian forces have withdrawn. 

In the northern city of Chernigiv, which was besieged from the early days of the invasion, a charred children's hospital, full of bullet and shrapnel holes, served as a shelter.

In the dank basement, children painted on walls -- tiny handprints, a smeared rainbow, a fluttering Ukrainian flag.

"Cluster bombs were flying, we have traces of these bombs," said 51-year-old Olena Makoviy. "The injured were brought to the children's hospital, both adults and children."

City officials estimate around 350 civilians have been killed in Chernigiv, with fellow residents digging mass graves to bury them. 

"It was very scary here from the first days of the war," said Makoviy. "They brought guys, handsome, young, but no longer alive,"

- 'We are ready' -
The Russian withdrawal from areas around Kyiv and the north is part of a shift of focus towards Ukraine's southeast, in a bid to create a land bridge between occupied Crimea and Moscow-backed separatist statelets in Donbas.

NATO chief Jens Stoltenberg said the alliance expects a Russian push in "coming weeks" to try to seize the entire Donbas. 

In the village of Krasnopillia there, Ukrainian forces were preparing to counter that push. 

"We know the Russians are reinforcing and are getting ready to attack," a senior Ukrainian officer on the ground told AFP. "We are ready... we've planned some surprises for them along the way."

Civilians have been asked to evacuate west and on Tuesday a line of cars stretching three kilometres was waiting to pass a checkpoint, while thousands of other residents boarded trains to leave.

And violence has continued elsewhere, with Ukraine's prosecutor general saying Tuesday that bombing around Kyiv had killed 12 people.

Peace talks between the sides have so far gone nowhere, though Moscow says it is "ready" to continue.

Ukraine has proposed an agreement where other countries would guarantee its security in return for Kyiv accepting a neutral and non-nuclear status, not joining NATO and refusing to host foreign military bases.

The proposal would also see Russia accept Kyiv's admission to the European Union.

Latest developments
Here are the latest developments in the war in Ukraine:

- 'Act immediately' -
Ukrainian President Volodymyr Zelensky challenges the United Nations to "act immediately" or "dissolve yourself altogether" during an address in which he shows harrowing footage of dead bodies -- including children -- he says were victims of Russian atrocities.

Likening Russia's actions in Bucha and other Ukrainian cities to violence carried out by "terrorists" such as the Islamic State group, Zelensky calls on the Security Council to expel Russia "so it cannot block decisions about its own aggression, its own war."

- Moscow denies atrocities -
Russian Foreign Minister Sergei Lavrov says the discovery of bodies in Bucha was a "provocation" aimed at scuppering talks between Moscow and Kyiv.

"A question arises: What purpose does this blatantly untruthful provocation serve? We are led to believe it is to find a pretext to torpedo the ongoing negotiations," Lavrov says in a video message broadcast on Russian television.

- US announces military aid -
Washington announces it will send $100 million in additional anti-armour weapons to Ukraine, bringing US military aid to Ukraine to over $1.7 billion, according to the Pentagon.

- 600,000 evacuated to Russia -
Russia's UN ambassador Vassily Nebenzia says over 600,000 people have been evacuated into Russia during the conflict in Ukraine, denying Kyiv's claim of mass deportations.

"And we're not talking about any kind of coercion or abduction, as our Western partners like to present this, but rather the voluntary decision by these people..." he tells the Security Council.

- Donbas onslaught feared -
NATO chief Jens Stoltenberg says that, after withdrawing most of its troops from northern Ukraine, Moscow aims to capture the "entire" Donbas region in the east, with the aim of creating a land corridor from Russia to annexed Crimea.

- Cluster munitions -
UN undersecretary-general for political and peacebuilding affairs, Rosemary DiCarlo, tells the Security Council of "credible" claims Russia has used indiscriminate cluster munitions two dozen times in populated parts of Ukraine.

- War chest frozen -
Britain has frozen some $350 billion (321 billion euros) in assets from the "war chest" of Russian President Vladimir Putin, British Foreign Secretary Liz Truss says during a visit to Warsaw.

She says this action meant that "over 60 percent of the regime's $604 billion foreign currency reserves" were now "unavailable" to the Russian government.

- US squeezes Russia payments -
The United States bars Moscow from making debt payments using funds held at American banks -- a move that takes Russia a step closer to default.

"Russia must choose between draining remaining valuable dollar reserves or new revenue coming in, or default," a Treasury spokesperson tells AFP.

On Wednesday, the US, G7 and EU are expected to ban all new investments in Russia in a new round of sanctions.

- EU to target Russian coal -
The EU proposes a ban on Russian coal imports and on Russian ships entering European ports.

"Russia is waging a cruel, ruthless war, also against Ukraine's civilian population. We need to sustain utmost pressure at this critical point," European Commission President Ursula von der Leyen says.

- Putin warning on food -
Putin retaliates by saying Moscow will "carefully monitor" food exports to "hostile" nations.

He cites "global food shortages" for the need in caution in exporting "to countries that are clearly hostile towards us."

- Russia government Twitter limits -
Twitter says it will introduce new measures against Russian government accounts to reduce the impacts of official propaganda.

Moscow restricts access to Twitter and has blocked Facebook and Instagram.

- Mariupol helicopters hit -
Russia says it shot down two Ukrainian helicopters that were trying to evacuate leaders of the controversial nationalist Azov battalion from besieged Mariupol.

It also says it proposed that Ukrainian fighters lay down their arms and leave the city "via an agreed route" to territory under Kyiv's control, adding that Ukraine had "ignored" the suggestion.