Russia has formally complained to the United States over its military aid to Ukraine, warning of "unpredictable consequences" if shipments of advanced weaponry go forward, US media reported.

In a diplomatic note this week, Moscow warned the United States and NATO against sending the "most sensitive" weapons for Kyiv to use in the conflict with Russia, saying such shipments were "adding fuel" to the situation and could come with "unpredictable consequences," the Washington Post reported.

The warning came the same week that US President Joe Biden pledged a new $800 million military aid package for Ukraine, including helicopters, howitzers and armored personnel carriers.

"What the Russians are telling us privately is precisely what we've been telling the world publicly -- that the massive amount of assistance that we've been providing our Ukrainian partners is proving extraordinarily effective," the Post quoted a senior administration official -- who spoke on condition of anonymity about the note -- as saying.

The State Department declined to comment on reports of the formal note.

"We won't confirm any private diplomatic correspondence," a State Department spokesperson said.

"What we can confirm is that, along with allies and partners, we are providing Ukraine with billions of dollars' worth of security assistance, which our Ukrainian partners are using to extraordinary effect to defend their country against Russia's unprovoked aggression and horrific acts of violence."

According to the New York Times citing US officials, the note was sent through normal channels, and was not signed by any senior Russian officials.

The formal correspondence indicates Russia is concerned about the United States' ongoing material support for Ukraine, an anonymous US official told CNN. 

CNN also reported that one source familiar with the document said the complaint could mean Moscow is getting ready to adopt a more aggressive stance against the United States and NATO as the invasion of Ukraine continues. 

Biden told his Ukrainian counterpart Volodymyr Zelensky of the new weapons aid package over the phone Wednesday, as Russia refocused its efforts eastward, the new frontline of the seven-week-old war.

"As Russia prepares to intensify its attack in the Donbas region, the United States will continue to provide Ukraine with the capabilities to defend itself," Biden said.

"This new package of assistance will contain many of the highly effective weapons systems we have already provided and new capabilities tailored to the wider assault we expect Russia to launch in eastern Ukraine."

Zelensky for his part tweeted that he and Biden had discussed the "additional package of defensive and possible macro-financial aid."

The new assistance included some of the heavier equipment that Washington had previously refused to provide to Kyiv for fear of escalating the conflict with nuclear-armed Russia, and comes after previous weapons aid already supplied to the Ukrainian army.

Latest developments

Here are the latest developments in the war in Ukraine:

- Russia threatens Kyiv -
A Ukrainian military factory outside Kyiv is partially destroyed by overnight Russian strikes, AFP reports, as Moscow warns it will intensify attacks on the capital in response to Ukrainian strikes on Russian soil.

Russian officials accuse Ukraine of sending two helicopters across the border to bomb a town in Russia's southern Bryansk region, wounding eight people. Ukraine has denied the accusations.

The military factory outside Kyiv produced missiles allegedly used to hit Russia's Moskva warship.

"The number and scale of missile strikes against targets in Kyiv will increase in response to any terrorist attacks or sabotage committed by the Kyiv nationalist regime on Russian territory," the defence ministry in Moscow says.

- Five million flee Ukraine -
More than five million people have now fled Ukraine since the Russian invasion on February 24, the United Nations says.

The UN's refugee agency lists 4,796,245 million Ukrainians who have crossed the borders, while its International Organization for Migration says nearly 215,000 third-country nationals have also left.

- Russian flagship sinks -
Russia's guided-missile cruiser Moskva sinks in the Black Sea after being damaged, Russia's defence ministry says.

Ukraine says Russia will seek revenge after the sinking of its flagship, which Kyiv claims to have hit with Neptune missiles. 

"The Moskva cruiser strike hit not only the ship itself: it hit the enemy's imperial ambitions. We are all aware that we will not be forgiven for this," Natalia Gumeniuk, a spokeswoman for Ukraine's southern military forces says.

- Finland NATO bid 'highly likely' -
Finland's European Affairs Minister Tytti Tuppurainen says it is "highly likely" that Finland will apply for NATO membership, just hours after Russia warns of unspecified "consequences" should it and Sweden join the military alliance.

Moscow's military actions in Ukraine have sparked a dramatic U-turn in public and political opinion in non-aligned Finland and Sweden, which is also discussing joining the alliance.

- 'Polish mercenaries' killed -
Russia's defence ministry says it has killed up to 30 Polish mercenaries fighting for Ukraine in the northeastern region of Kharkiv.

It says strategic rocket forces "eliminated up to 30 Polish mercenaries" in a strike on the village of Izyumskoe, near the city of Kharkiv.

- Kharkiv offensive -
Russian strikes on the eastern Ukrainian city of Kharkiv killed seven people and left another 34 injured, the region's governor says, as Moscow's forces stepped up attacks.

"The occupiers fired on a residential area in Kharkiv. Unfortunately, 34 people were injured, including three children. Seven people died; one among the dead was a child," Oleg Synegubov says on social media.

- EU gas, oil embargo will take 'months' -
The EU is working on broadening sanctions on Russia to include oil and gas embargoes but such measures will take "several months",  European officials tell AFP.

Russian energy exports are Moscow's main hard currency earner, and its oil and gas sales to the EU account for between a quarter of a billion to a billion euros per day, per different estimates.

- Russia kicks out EU diplomats -
Moscow says 18 members of the EU mission in Russia have been declared "persona non grata" and must leave the country.

- French radio RFI site blocked -
Russia's media watchdog Roskomnadzor blocks access to the website of French radio station RFI for violating a law banning the dissemination of false or extremist information.

It does not specify how the radio station has fallen foul of the law.