Ukrainian and pro-Russian forces flexed their military muscles in the restive east of the country on Wednesday, a day ahead of high-level diplomatic talks on the escalating crisis.

Armored vehicles from the rival sides appeared on the streets of two neighboring towns after Russian President Vladimir Putin warned that Kiev's decision to send in troops to put down a separatist uprising in its industrial heartland had dragged the country to the brink of civil war.

Pro-Russia separatists on Wednesday seized six armored personnel carriers from Ukrainian armed forces with the help of Russian agents, Ukraine's defense ministry said.

"A column was blocked by a crowd of local people in Kramatorsk with members of a Russian diversionary-terrorist group among them," the statement said. "As a result of the blocking, extremists seized the equipment."

The statement said the troop carriers were now in the town of Slavyansk, guarded by "people in uniforms who have no relation to Ukraine's armed forces."

As the situation on the ground appeared to escalate, authorities in Kiev ratcheted up the verbal attack on Russia, with Prime Minister Arseniy Yatsenyuk accusing Moscow of trying to build "a new Berlin wall."

Yatsenyuk demanded Moscow halt its alleged support for the separatists but said Kiev remained committed to Thursday's crunch talks between the top diplomats of Russia, the EU, the US and Ukraine.

Ukraine's military also pledged a firm response after two servicemen were allegedly taken hostage by pro-Russian forces in the region of Lugansk.

The Ukrainian Security Service (SBU) said in a statement that Russian commanders in the east had issued pro-Kremlin militants with "shoot-to-kill" orders.

Russia is deliberately keeping the pressure high on Ukraine, but its aim is to transform Ukraine into a federation which maintains neutrality instead of splitting the country, Xia Yishan, a research fellow with the China Institute of International Studies, told the Global Times.

NATO said Wednesday it planned to deploy more forces in Eastern Europe in the face of the crisis.

"You will see deployments at sea, in the air, on land to take place immediately, that means within days," NATO Secretary-General Anders Fogh Rasmussen told a news conference in Brussels.

Jay Carney, a spokesman for US President Barack Obama, said Tuesday Ukraine's government was obliged to respond to "provocations" in the east, but Washington was not considering sending arms to Kiev.