The US said it has called for an open meeting of the UN Security Council on Monday to address the crisis around Ukraine because of "threatening behaviour" by Russia.
"More than 100,000 Russian troops are deployed on the Ukrainian border and Russia is engaging in other destabilising acts aimed at Ukraine, posing a clear threat to international peace and security and the UN Charter," Washington's UN envoy Linda Thomas-Greenfield said on Thursday.
"As we continue our relentless pursuit of diplomacy to de-escalate tensions," she said, the 15-member Security Council "must squarely examine the facts and consider what is at stake for Ukraine, for Russia, for Europe, and for the core obligations and principles of the international order should Russia further invade Ukraine".
With fears rising that Russia could invade its neighbour, a former Soviet state, Ms Thomas-Greenfield said the council faced a "crucial" matter for international peace and security.
"Russia's threatening behaviour against Ukraine and the build-up of Russian troops on Ukraine's borders and in Belarus," she said.
"This is not a moment to wait and see. The Council's full attention is needed now and we look forward to direct and purposeful discussion on Monday."
As one of the five permanent members of the Security Council, Russia has the power of veto on any UN resolution brought to the council.
But Ms Thomas-Greenfield said the meeting would be more of an opportunity to "expose Russia" for its action and isolate the Kremlin for its aggressive posture on Ukraine.
"And while they do have the veto power, their isolation will be felt if the Security Council brings this, and we present a united front against the Russians," she told Romanian Public Television.
"I don't think any country in the Security Council will sit back and say it's OK for Russia to invade the borders of another country."
Originally the US had hoped to hold the Security Council meeting on Friday, diplomats said.
But it agreed to push it to Monday so as not to interfere with a scheduled Friday phone call between French President Emmanuel Macron and his Russian leader Vladimir Putin.