Russian President Vladimir Putin warned that Ukraine is nearing a civil war, after the central government deployed troops to target pro-Russia separatist militia, while Kiev stated that pro-Russian forces have a “shoot to kill” order.

“The sharp escalation of the conflict puts the country, in essence, on the verge of a civil war,” Putin said in a phone conversation with German Chancellor Angela Merkel, Russia’s Itar-Tass news agency reported.

The statement raised concerns in Kiev and the West that Russia’s military might intervene to “protect” Russian speakers in eastern Ukraine, after the annexation of the Crimean region in March.

Ukraine’s Western-backed prime minister accused Russia Wednesday of erecting a new “Berlin Wall,” a move which threatens European security.

“Today’s events... are starting to endanger Europe and the European Union. It is now clear that our Russian neighbors have decided to build a new Berlin Wall and return to the Cold War era,” Arseniy Yatsenyuk told a government meeting, Agence France-Presse reported.

The top official also accused Russia of “exporting terrorism” to Ukraine through covert forces aimed at organizing armed separatists. He called on the neighboring country to “immediately” terminate its actions.

“The Russian government must immediately call off its intelligence-diversionary groups, condemn the terrorists and demand that they free the buildings,” Yatsenyuk said.

“That is, if the Russian Federation is interested in stabilizing the situation, which I have significant doubts about,” he added.

Russia’s foreign ministry expressed “deep concern” at what it claimed were reports of casualties in eastern Ukraine, but casualties have not been confirmed.

Kiev launched a “special operation” against separatists in Russian-speaking eastern Ukraine, but the operation was limited to soldiers landing from two helicopters in Kramatorsk. Troops then withdrew into barracks after locals at a barricade were hostile to them.

Acting President Oleksander Turchinov in Kiev said the air base was “liberated,” declaring victory over pro-Russian rebels. There was no sign of militants there, Reuters reported.

Earlier that day, a senior Ukrainian official told a crowd he had come to direct an “anti-terrorist operation,” more than a week after a missed deadline set by Kiev for the armed activists to end an occupation of public buildings in around 10 areas in the east.

A spokesman for U.S. President Barack Obama said Ukraine’s government must respond to “provocations” from the east, adding that Washington was not considering sending arms to Kiev.