• U.S. Ambassador Linda Thomas-Greenfield told an emergency meeting of the UN Security Council that the U.S. will unveil new sanctions against Russia on Tuesday.
• On Monday, President Joe Biden signed an executive order that prohibits new investment, trade and financing in the so-called Donetsk People's Republic or the Luhansk People's Republic regions of Ukraine.
• The State Department relocated its diplomatic staff out of Ukraine to Poland on Monday evening.

The United Nations Security Council held a rare emergency meeting on Monday night in New York to address the escalating Russian military deployment to Ukraine.

The long-simmering conflict entered a new phase Monday when Russian President Vladimir Putin declared that Russia would recognize two breakaway regions of Eastern Ukraine as "independent republics," and that Moscow would deploy "peacekeeping troops" to the two newly "independent" territories.

U.S. Ambassador Linda Thomas-Greenfield said Putin's announcement posed a direct threat not merely to Ukraine, but to every other sovereign member state of the United Nations.

"President Putin is testing our international system. He is testing our resolve and seeing just how far he can push us all," she said. "He wants to demonstrate that through force, he can make a farce of the UN. We must act together in response to this crisis."

Greenfield said the United States would unveil new sanctions against Russia on Tuesday, and urged member states to impose severe consequences on Moscow.

"An attack on Ukraine is an attack on the sovereignty of every UN member state, and the UN charter," she said.

The last minute Security Council meeting followed a day of swiftly escalating crises, as Washington raced to prepare for what appeared inevitable and Europe braced for what Biden predicted in January would be "the largest invasion since World War II."

Stock futures fell sharply on Monday night while oil prices rose as traders continued to monitor the growing tensions.

It was unclear Monday evening what impact, if any, the coming U.S. sanctions against Russia might have on financial markets or oil prices.

On Monday, President Joe Biden signed an executive order that prohibits new investment, trade and financing by U.S. persons to, from, or in the so-called Donetsk People's Republic or the Luhansk People's Republic regions of Ukraine.

That action was in response to Putin's declaration that Russia would recognize the two breakaway regions of Ukraine as separate countries. But White House officials declined Monday to specify what might be in the next round of sanctions coming on Tuesday.

Biden also convened a meeting of his National Security Council, the second such meeting in as many days. Later in the day, Biden spoke to Ukrainian President Volodymyr Zelenskyy, and he also held a joint call with French President Emmanuel Macron and German Chancellor Olaf Scholz.

White House officials said the U.S. had discussed plans with the Ukrainian government to relocate Zelenskyy to Lviv, Ukraine from Kyiv if Russian forces launched an assault on the capital city — a scenario that Biden recently said American officials believe is likely.

Meanwhile, the State Department relocated its own diplomatic staff out of Lviv and over the border to Poland on Monday evening.

"Our personnel will regularly return to continue their diplomatic work in Ukraine and provide emergency consular services," Secretary of State Antony Blinken wrote in a statement announcing the update.

Blinken reiterated his call for Americans to depart Ukraine immediately, adding that the security situation remains unpredictable and may deteriorate with little notice.

This is a developing news story, please check back for updates.

— CNBC's Amanda Macias contributed reporting.