New York, New York - The United States and Russia aired their grievances at the UN Security Council on Monday, in the first open session the body has held to discuss the Ukraine crisis.

The US ambassador stressed the size of the Russian troop build-up along Ukraine's border, while the Russian ambassador accused Washington of fuelling war hysteria.

The back-and-forth over Ukraine lasted some two hours and featured repeated clashes as the diplomats took positions that have become familiar after weeks of talks and escalating tensions.

"Russia has assembled a massive military force of more than 100,000 troops along Ukraine's border. These are combat forces and special forces prepared to conduct offensive actions into Ukraine," said Linda Thomas-Greenfield, US ambassador to the UN.

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"This is the largest – this is the largest; hear me clearly – mobilization of troops in Europe in decades. And as we speak, Russia is sending even more forces and arms to join them."

"If Russia further invades Ukraine, none of us will be able to say we didn't see it coming," Thomas-Greenfield said, describing the consequences of such an assault as "horrific."

Russian Ambassador Vasily Nebenzya said the US was trying to "whip up hysterics" and denounced what he called the Biden administration's "megaphone diplomacy."

"The discussions about an imminent threat of war are provocative in and of themselves. They almost call for it. They want it to happen," Nebenzya said, adding that Russian officials had categorically denied accusations that it was planning an invasion – "and I will do so now [as well]."

The US requested the meeting in the Security Council because it and its Western allies fear a fresh Russian assault on Ukraine.

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Russia failed to get Monday's meeting cancelled, with 10 of the 15 member states voting in favor of deliberations in New York.

Russia says it is only conducting military maneuvers that happen to be near the border and says it has no designs on Ukraine. At the same time, it is demanding that NATO promise not to take in more members from Eastern Europe and to keep military forces out of Eastern European countries already in the bloc.

After the UN meeting, the White House defended its escalating rhetoric amid the accusations of scaremongering, including by Kyiv.

"We feel it's important to be open and candid about the threats from Russia. It's not just words of course, you are seeing specifics that we have been laying out here," White House spokesperson Jen Psaki said.

She said it was a fact that Russia had amassed 100,000 troops and military equipment on Ukraine's border and was also "surging" forces into neighboring Belarus.

"Our effort is to ensure we are informing the American public and the global community of the seriousness of this threat," she said.

Ukrainian President Volodymyr Zelensky late last week accused the US of being too alarmist and downplayed Washington's assessment of the situation, remarks that showed Kyiv's frustration with its main ally.

"As soon as the White House realizes that there are certain risks, they keep talking about them. In my opinion, this is a mistake because the world reacts very strongly to it."

UK Prime Minister Boris Johnson and Polish Prime Minister Mateusz Morawiecki were due in Ukraine on Tuesday as they personally step up their diplomatic efforts.

A planned telephone call between Johnson and Russian President Vladimir Putin was postponed on Monday as the British PM deals with the fallout from "partygate". Downing Street had earlier said that Johnson would tell Putin to "take a diplomatic path" and avoid "very costly" military action in Ukraine.

However, Putin did speak to French President Emmanuel Macron on Monday, a follow-up to a call they had on Friday to find a way to de-escalate tensions. The two were now exploring the possibility of meeting in person, the Kremlin said.