The U.N. Security Council met in an emergency late night session Monday after Russia's president signed a declaration recognizing two breakaway regions of Ukraine as independent states.

"The internationally-recognized borders of Ukraine have been and will remain unchangeable, regardless of any statements and actions by the Russian Federation," Ukraine's U.N. ambassador Sergiy Kyslytsya told the council of President Vladimir Putin's recognition of the eastern regions of Donetsk and Luhansk.

"We demand from Russia to cancel the decision on recognition and return to the table of negotiations," he said. "We condemn the order to deploy additional Russian occupation troops in the territories of Ukraine. We demand immediate and complete, verifiable withdrawal of the occupation troops."

The United States, the European Union and Britain have all announced they are imposing a first round of sanctions on Moscow for the move, which violates international norms and law and Ukraine's sovereignty. In the Security Council, they reaffirmed their commitment to Ukraine's sovereignty.

"In this moment, no one can stand on the sidelines," U.S. Ambassador Linda Thomas-Greenfield said. "We must make it clear that an attack on Ukraine is an attack on the sovereignty of every U.N. member state and the U.N. Charter - and that it will be met with swift and severe consequences."

She said Washington continues to believe that diplomacy is the only way to resolve the crisis and preserve peace.

FILE - U.S. Ambassador to the United Nations Linda Thomas-Greenfield speaks to reporters during a news conference at United Nations headquarters.

Western leaders and analysts say the move is a pretext for Russia to invade its southern neighbor, something Moscow has denied until now as western "hysteria."

President Putin followed recognition with an order for what he called "peacekeeping" troops to deploy to the two Russian-occupied regions to protect civilians there. Since 2014, Moscow has given citizenship to several hundred thousand people who are of Russian ethnicity in Donetsk and Luhansk.

"He calls them peacekeepers, this is nonsense," the U.S. envoy said. "We know what they really are."

Western officials say Moscow has amassed more than 150,000 troops on Ukraine's borders.

"The next hours and days will be critical," U.N. political chief Rosemary DiCarlo warned. "The risk of major conflict is real and needs to be prevented at all costs."

Russia's envoy said from listening to other council members, one might get the impression that Moscow's decision was sudden and rash.

"Of course, that's not the case. It should be remembered that the DPR and LPR declared their independence from Ukraine back in 2014," Vassily Nebenzia said, using an abbreviation for Donetsk and Luhansk. "But we only recognized them now, despite the high level of support in both in the republics and in Russian society from the very beginning."

He said 60,000 people have fled the area into Russia in recent days, blaming intensified Ukrainian shelling. Kyiv says it is not attacking the population.

The Organization for Security Cooperation in Europe's (OSCE) Special Monitoring Mission recorded more than 3,000 cease-fire violations in the region from February 18-20, including 926 explosions in Luhansk region, and 1,100 blasts in Donetsk. It did not attribute cause or source.

"We remain open to diplomacy, for a diplomatic solution," Nebenzia said. "However, allowing a new bloodbath in the Donbas is not something we intend to do," he said referring to the collective name of the region.

Council members overwhelmingly expressed their support for Ukraine's sovereignty, territorial integrity and political independence, while calling for a diplomatic solution.

"Russia has brought us to the brink," British envoy Barbara Woodward said. "We urge Russia to step back."

Kenya's envoy said his country is "gravely concerned" by Moscow's recognition of Donetsk and Luhansk. He said that while nearly every African country had been born out of the end of an empire, with artificial borders drawn by colonial powers, they kept them because they followed the U.N. Charter and they wanted peace.

"We believe that all states formed from empires that have collapsed or retreated have many peoples in them yearning for integration with peoples in neighboring states. This is normal and understandable. After all, who does not want to be joined to their brethren and to make common purpose with them?" he asked.

"However, Kenya rejects such a yearning from being pursued by force. We must complete our recovery from the embers of dead empires in a way that does not plunge us back into new forms of domination and oppression," Ambassador Kimani said.

Ukraine is a former republic of the Soviet Union, but gained its independence in a referendum in 1991.