The US ambassador to the UN, Linda Thomas-Greenfield, told the Security Council on Monday that Russia's military build-up along its border with Ukraine was the "largest mobilisation of troops in Europe in decades" and urged members to act against the crisis.
Ms Thomas-Greenfield said Russia's deployment of "combat forces and special forces" was poised to "conduct offensive operations into Ukraine" in one of the tensest European stand-offs since the Second World War.
"This is the largest — hear me clearly — mobilisation of troops in Europe in decades," Ms Thomas-Greenfield said in New York.
"As we speak, Russia is sending even more forces and arms to join them."
A further 5,000 troops had been deployed to Belarus, which also neighbours Ukraine, including "missiles, special forces and anti-aircraft batteries", she added.
The US called the public UN meeting after several rounds of private talks between Russia, the US and others, had made only minimal progress and to offer Moscow another "diplomatic path out of the crisis", said Ms Thomas-Greenfield.
"Today in the United Nations, we laid out the full nature of Russia's threat to Ukraine's sovereignty and territorial integrity of Ukraine, as well as (to the) core tenets of the rule-based international order," US President Joe Biden said on Monday.
"We continue to urge diplomacy as the best way forward, but with Russia continuing its build-up of its forces around Ukraine, we are ready no matter what happens."
Moscow insists that it has no intention to attack Ukraine for a second time.
Russia's UN ambassador Vasily Nebenzya said Washington was trying to "whip up hysterics" about the "myth of Russian aggression" by exaggerating the size of its troop build-up.
He blamed the US for the 2014 ouster of a Kremlin-friendly president in Kiev, saying it brought to power “nationalists, radicals, Russophobes and pure Nazis,” and created the antagonism that exists between Ukraine and Russia.
“If they hadn’t done this, then we to date would be living in a spirit of good neighbourly relations and mutual cooperation,” Mr Nebenzia said.
“However, some in the West just don’t clearly like this positive scenario. What’s happening today is yet another attempt to drive a wedge between Russia and Ukraine.”
Mr Nebenzia pointedly left the council chamber as the Ukrainian Ambassador Sergiy Kyslytsya started to speak.
“How long Russia will pressure, will pursue a clear attempt to push Ukraine and its partners into a Kafka trap?” Mr Kyslytsva asked.
Any action by the council was unlikely, as Russia, together with its frequent council ally China, and other permanent members can block statements and resolutions from the chamber.
A Russian effort to stop the meeting from going ahead did not garner enough votes.
China joined Russia in voting against a public session. India, Kenya and Gabon abstained. The remaining 10 members supported an open meeting.
Chinese ambassador Zhang Jun said the disagreement called for "quiet diplomacy", not a public meeting.
The talks included Ukraine, Belarus, Poland and Lithuania.
Russia’s massing of more than 100,000 troops along its border with Ukraine has brought increasingly strong warnings from the West that Moscow plans to attack, kick-starting several rounds of high-stakes diplomacy reminiscent of the Cold War.
US Secretary of State Antony Blinken was preparing for talks with his Russian counterpart Sergei Lavrov on Tuesday — the latest diplomatic contact between Moscow, Washington and Brussels over Ukraine, and wider European security concerns.
Moscow's demands include Nato pledging never to allow Ukraine to join the alliance, a halt to deployments of Nato weapons along Russia’s borders and a withdrawal of forces from Eastern Europe — demands that the organisation and the US say are not possible.
The US and its allies have ramped up joint efforts to deter any invasion of Ukraine, warning such a move would be met with new and "devastating" economic sanctions.
US President Joe Biden on Monday said in a statement that the West was "clear-eyed about the actions Russia is threatening" and was ready "for every scenario".
"Today’s Security Council meeting is a critical step in rallying the world to speak out in one voice: rejecting the use of force, calling for military de-escalation, supporting diplomacy as the best path forward," he said.
The UAE's ambassador to the UN Lana Nusseibeh urged those involved to negotiate a way out of the crisis and ensure "stability and peace" in the region.
The council has met frequently to discuss Ukraine, notably in 2014 after Russia annexed the country's Crimea region. In March that year, Russia vetoed a US-drafted resolution opposing a referendum on the status of the peninsula. China abstained.
Russia is set on Tuesday to assume the council’s rotating presidency for February. It is a largely administrative role, but Russian diplomats may be able to slow-walk requests from other council members on the Ukraine crisis for the month.
The UAE, Albania, Brazil, Gabon and Ghana joined the UN council for two-year terms beginning on January 1, meaning they can take part in meetings, vote on resolutions and help draft official statements.
Agencies contributed to this report.