U.S. Vice President Joe Biden will travel to Ukraine later this month, officials said Saturday, as Washington warned Moscow of "additional consequences" if it fails to reduce tensions with Ukraine.

The announcement came just a day after Washington unveiled sanctions against six of Crimea's breakaway leaders, including the official who signed the deal with Moscow to split the peninsula from Ukraine.

During a telephone call with Russian Foreign Minister Sergei Lavrov, U.S. Secretary of State John Kerry "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," a senior State Department official said.

Washington has repeatedly urged Moscow to de-escalate tensions and withdraw Russian troops from Ukraine's eastern border in the worst East-West standoff since the Cold War.

A first wave of U.S. sanctions unveiled in March had blacklisted officials and businesspeople close to Russian President Vladimir Putin to protest Moscow's takeover of Crimea.

The senior official said that Kerry expressed "strong concern" that earlier attacks by armed militants in eastern Ukraine were "orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea."

Kalashnikov-wielding gunmen seized two security buildings in Ukraine's restive eastern rust belt amid spreading protests demanding the Russified region join Kremlin rule.

The militants were equipped with specialized Russian weapons and wore the same uniforms as the Russian forces who invaded the Crimean peninsula, according to the official.

The White House condemned the "orchestrated campaign of incitement and sabotage," saying it would figure in Biden's April 22 talks in Kiev.

"The vice president will discuss the latest developments in eastern Ukraine, where pro-Russian separatists, apparently with the support of Moscow, continue an orchestrated campaign of incitement and sabotage to destabilize the Ukrainian state," it said.

Biden's visit to Kiev will also focus on the international community's efforts to help stabilize and strengthen Ukraine's economy ahead of May 25 presidential elections, the White House said.

And he will discuss constitutional reform, decentralization, anti-corruption efforts and energy security.

Moscow has massed tens of thousands of troops on Ukraine's eastern border after annexing the Crimean Peninsula and nearly doubled the rates it charges Kiev for natural gas.

During his meetings with government leaders and civil society groups, Biden will "underscore the United States' strong support for a united, democratic Ukraine that makes its own choices about its future path," a White House statement said.

"The vice president will also meet with various Ukrainian people to hear their aspirations and deepen the partnership between the United States and Ukraine." (AFP)