UNITED NATIONS — The U.N. Security Council met late Sunday in emergency session as violence escalated in eastern Ukraine, hours before a Monday morning deadline for pro-Russia protesters to lay down their arms or face Ukrainian troops.

Russia called the meeting shortly after Ukrainian special forces exchanged gunfire Sunday with a pro-Russia militia in an eastern city, and at least one security officer was killed and five others wounded. Ukraine’s president accused its powerful neighbor of fomenting unrest, and announced that his government would deploy armed forces Monday to quash an increasingly bold pro-Russian insurgency. 

“At this moment, Ukraine teeters on the brink,” Assistant U.N. Secretary-General Oscar Fernandez-Taranco told Security Council members.

Ethnic Russians in Ukraine’s east fear that the country’s new pro-Western government will oppress them, and are demanding to have referendums on autonomy and possible annexation by Russia. 

Fernandez-Taranco said U.N. monitors in eastern Ukraine have described seeing pro-separatist protesters as being armed with machine guns and sniper rifles.

“The fact is that many of the armed units that we’ve seen were outfitted in bulletproof vests, camouflage uniforms with insignia removed,” U.S. Ambassador Samantha Power said. “These armed units … raised Russian and separatist flags over seized buildings and have called referendums and union with Russia. We know who is behind this.”

Russia has tens of thousands of troops massed along Ukraine’s eastern border, and there are fears that Moscow might use the violence in the mainly Russian-speaking region as a pretext for an invasion, in a repeat of events in Crimea weeks ago.

“This is not a war between Ukrainians, this was artificially created,” said Ukraine’s U.N. Ambassador Yuriy Sergeyev. 

Russian Ambassador Vitaly Churkin denied Western and Ukrainian claims that Moscow was behind the violence, and told U.N. diplomats that Ukraine has been using radical neo-Nazi forces to destabilize its eastern region.

“It is the West that will determine the opportunity to avoid civil war in Ukraine. Some people, including in this chamber, do not want to see the real reasons for what is happening in Ukraine and are constantly seeing the hand of Moscow in what is going on,” Churkin said. “Enough. That is enough.”

He said after the meeting that he hoped Western powers would pressure Ukraine to rethink its deadline for sending in troops.

“Whether they are going to put an end to this provocation by Kiev, this is their responsibility to prevent further escalation of this crisis,” Churkin said.