The United States and EU were readying new sanctions against Russia on Wednesday, after Ukraine's president showed the UN Security Council harrowing images of violence and accused Moscow of widespread atrocities.

The measures follow an international outcry over hundreds of civilians found dead in areas from which Russian troops have withdrawn around Ukraine's capital, including the town of Bucha.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 night-time 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 ambassador Linda Thomas-Greenfield 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. 

Washington, in coordination with the G7 and the European Union, is expected on Wednesday 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. 

The EU meanwhile was poised to implement a fifth round of sanctions cutting off Russian coal imports.

European Council chief Charles Michel said the bloc must also impose oil and gas sanctions on Russia "sooner or later".

The deaths of civilians in Bucha and other parts of Ukraine were "war crimes", he said, adding: "There must be, and there will be, severe consequences for all those responsible."

However, addressing the Irish parliament Wednesday, Zelensky condemned the "indecisiveness" on the part of EU nations, which are dependent on Russian energy.

"We still need to convince Europe that Russian oil cannot feed the Russian military machine with new sources of funding," Zelensky added, calling also for the total exclusion of Russian banks from Western finance.

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

In a further effort to isolate Moscow, a string of EU countries including Germany, France, Italy and Spain have expelled more than 200 Russian diplomats and staff between them this week.

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.