(Reuters) - Armed separatists took control of a city in eastern Ukraine on Saturday and Kiev prepared troops to tackle what it called an "act of aggression by Russia," pushing the conflict between the neighbors into a dangerous new phase.

Pro-Russian activists carrying automatic weapons seized government buildings in Slaviansk, a town about 150 km (90 miles) from the Russian border, and set up barricades on the outskirts of the city.

In Kramatorsk, some 80 km to the north, gunmen seized the police station after a shootout with police, a Reuters witness said.

Government buildings in several other towns in the Donetsk and Luhansk regions were attacked in what Washington said were moves reminiscent of the events that preceded Russia's annexation of Ukraine's Crimean peninsula.

"We are very concerned by the concerted campaign we see under way in eastern Ukraine today by pro-Russian separatists, apparently with support from Russia," said Laura Lucas Magnuson, spokeswoman for the White House National Security Council.

"We saw similar so-called protest activities in Crimea before Russia's purported annexation.

"We call on President (Vladimir) Putin and his government to cease all efforts to destabilize Ukraine, and we caution against further military intervention."

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