Ukraine's president urged the United Nations Security Council on Tuesday to take immediate action against Moscow, calling for "accountability" for atrocities against civilians, as fears grow that Russia is preparing new offensives to seize territory in the east and south.

People "were killed in their apartments, houses ... civilians were crushed by tanks while sitting in their cars in the middle of the road," Volodymyr Zelenskyy said in a video message to the U.N. council in New York.

"Accountability must be inevitable," he said while calling for Russia's exclusion from the Security Council after six weeks of heavy bombardments of Ukraine.

"Are you ready to close the U.N.? And the time of international law is gone? If your answer is no, then you need to act immediately," he said.

His address came after global outrage over the harrowing discoveries of civilian victims in Bucha and other towns near Kyiv after Russian troops pulled back, which Zelenskyy and other officials have denounced as war crimes and attempted "genocide".

"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," U.S. Secretary of State Antony Blinken said before leaving for a NATO meeting in Europe starting Wednesday.

Washington and the EU have promised more sanctions to squeeze Russia's economy and force President Vladimir Putin to halt the war he launched six weeks ago, purportedly to defend pro-Russia enclaves in Ukraine's east.

"In the coming weeks, we expect a further Russian push in the eastern and southern Ukraine to try to take the entire Donbas and to create a land bridge to occupied Crimea," NATO chief Jens Stoltenberg said.

The EU announced the fifth package of measures that would target oil and coal exports and prohibit Russian ships from European ports, while the U.S. Treasury said Russia would no longer be able to pay its foreign debt with dollars held in American banks.

EU Commission chief Ursula von der Leyen, who said she would travel to Kyiv this week, has offered the bloc's assistance in documenting proof of war crimes.

The Kremlin has denied any civilian killings and claimed that the images are fakes produced by Ukraine forces, or that the deaths occurred after Russian soldiers pulled out of the areas.

But one resident in Bucha, Olena, told Agence France-Presse (AFP) she saw Russian soldiers shoot a man in cold blood as units of "brutal" older troops sowed fear in the town.

"Right in front of my eyes, they fired on a man who was going to get food at the supermarket," said the 43-year-old, who did not wish to give her family name.

In response, Spain, Italy, Denmark and Slovenia expelled dozens of Russian diplomats suspected of being intelligence operatives, after France and Germany did the same on Monday, for a total of some 180 expulsions in just 48 hours.

Putin warned of reprisals for what the Kremlin called a "short-sighted move" that would complicate efforts to negotiate an end to the hostilities.

He also said Moscow would "monitor" its food exports to "hostile" nations, raising the specter of further inflation pressures worldwide as the conflict endures.

Europe's worst conflict in decades has killed as many as 20,000 people, according to Ukrainian estimates.

Nearly 4.25 million Ukrainians have fled the country during Russia's invasion, while a further 7.1 million are thought to be internally displaced within Ukraine, the United Nations said Tuesday.

Many in Ukraine are bracing for further Russian bombardments, especially in the east and south, and air raid sirens rang out overnight across much of the country.

The full nature of the killings in Bucha and other areas from which Russian troops have withdrawn is still being pieced together.

Ukrainian officials say over 400 civilian bodies have been recovered from the wider Kyiv region, many of whom have been buried in mass graves.

But Zelenskyy has warned that the deaths in Bucha could be only the tip of the iceberg, saying he had information that even more people had been killed in places like nearby Borodianka.

AFP reporters who briefly visited the area saw no bodies in the streets, but locals reported many deaths. The scale of devastation in the town saw buildings flayed open.

"I know five civilians were killed," said 58-year-old Rafik Azimov. "But we don't know how many more are left in the basements of the ruined buildings after the bombardments."

"I buried six people," another resident, Volodymyr Nahornyi, said. "More people are under the ruins."

Even where troops have withdrawn, fears remain, with Kyiv's Mayor Vitali Klitschko telling residents to not return yet, citing the dangers of continued shelling and unexploded munitions.

On Monday, officials in Mykolaiv, on the Black Sea not far from Odessa, said cluster bombs were used against the city in strikes that killed 10 civilians and wounded 46.

Such weapons are banned under a 2008 U.N. convention that cites the danger of indiscriminate killing they pose by sending dozens of small bomblets over a large area, but it has not been signed by Russia or Ukraine.

The U.N.'s undersecretary-general for political and peacebuilding affairs, Rosemary DiCarlo, told the Security Council meeting there were "credible allegations" that Russia had used cluster munitions in populated areas at least 24 times.

Elsewhere in the south, concerns remain for civilians trapped in the city of Mariupol, which has been besieged by Russian forces for over a month, and where authorities say at least 5,000 people have been killed.

Mayor Vadym Boichenko said the city was now "unlivable" for the approximately 120,000 residents that have remained despite persistent Russian shelling, characterizing the situation as "beyond a humanitarian disaster."