WASHINGTON, April 12 -- U.S. Secretary of State John Kerry on Saturday voiced "strong concerns" about fresh attacks by armed militants in eastern Ukraine, threatening "additional consequences" unless Russia moves to ease tension there.

In a phone conversation with his Russian counterpart Sergei Lavrov, Kerry said the attacks earlier in the day "were orchestrated and synchronized, similar to previous attack in eastern Ukraine and Crimea," the State Department said in a statement.

About a dozen gunmen seized a police station and a state security building in Slavyansk, a town about 100 km north of Donetsk. Ukraine's police said the gunmen seized at least 400 handguns and 20 automatic weapons in the police station.

"The secretary made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences," the State Department statement said.

Lavrov told Kerry that "if Kiev's threats to use force against people driven to despair in the southeast are carried out, prospects for further cooperation on the Ukrainian issue, including a planned four-party meeting in Geneva, will be foiled," the Russian Foreign Ministry said.

The meeting between the EU, the U.S., Russia and Ukraine are scheduled for April 17. U.S. Vice President Joseph Biden will travel to Kiev on April 22, the White House said on Saturday.

A new wave of unrest erupted in eastern Ukraine last weekend, when pro-Moscow activists seized several government buildings in the cities of Donetsk, Lugansk and Kharkov, demanding referenda on autonomy and closer ties with Russia.

The developments came after Russian President Vladimir Putin and Crimean leaders signed a treaty on March 18 to make Crimea part of Russian territory, as almost 97 percent of voters on the peninsula backed secession from Ukraine in a referendum.

Kiev has rejected the referendum and Crimea's integration into Russia, saying it was unconstitutional.

In response, Washington has suspended military cooperation and trade and investment talks with Russia, and slapped sanctions on a number of key Russian and Crimean officials. It has threatened to target Russia's key sectors like energy, banking and mining in response to its further moves.