UNITED NATIONS, April 13 -- The situation in Ukraine has seriously deteriorated, a top UN official said in an emergency session of the UN Security Council on Sunday night, calling the crisis "more combustible than ever."

"There have been at least five new cities in eastern Ukraine targeted by uprisings and seizures of government buildings in the last 24 hours," said Assistant UN Secretary-General for Political Affairs Oscar Fernandez-Taranco, who briefed the council at a formal meeting requested by Russia.

The council meeting was held only hours before a deadline set by Kiev for pro-Russia separatists to disarm by Monday morning or face a "full-scale military operation."

"UN monitors have reported a consistent reinforcement of barricades and armed civilians, on both sides," Fernandez said, adding that machine guns and sniper rifles were sighted.

The exact number of casualties of the skirmishes remains unknown, he said, calling for "maximum restraint" on all sides and constructive dialogue between Kiev and Moscow to de-escalate the situation and address all differences.

Also at the meeting, Russian ambassador Vitally Churkin rebuffed the claims that Moscow was behind the violence, and said Kiev needs to stop the war with its own people.

Churkin also urged the European Union and the United States to "disassociate from the neo-Nazis" and stop the use of force against the Ukrainian people.

"It is the West that will determine the opportunity to avoid civil war in Ukraine," he said.

U.S. ambassador Samantha Power said it is Russia rather than the United States that had escalated the situation.

"It is hard to reconcile the behavior, the propaganda and the military actions of the Russian Federation ... with this appeal for diplomacy and de-escalation," she said.

China said it hoped relevant parties to keep calm, exercise restraint, and avoid further deterioration of the situation, and called for political and diplomatic settlement of the issue.