The United States will work on greater diversification of energy sources in the Baltics and may consider rotating units of its ground and naval forces through the region for training exercises, U.S. Vice President Joe Biden said on Tuesday.

Biden, who was speaking after meeting Estonian President Toomas Hendrik Ilves in the Polish capital, also said that his country remains committed to NATO's pledge to help member states if military action is taken against them.

Ilves said during a joint news conference with Biden that the European Union's response to Russia's intervention in Ukraine must be more robust and "should not be about the price of gas".

Ukraine warned on Tuesday its conflict with Russia had entered a "military stage" and authorised its troops to shoot in self-defence after both sides suffered their first casualites since pro-Kremlin forces seized Crimea nearly three weeks ago.
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The dramatic escalation to the raging security crisis on the EU's eastern frontier came hours after President Vladimir Putin signed a treaty claiming Crimea as Russian territory after the Black Sea region overwhelmingly voted in favour of switching from Ukrainian to Kremlin rule.

Ukraine's Western-backed Prime Minister Arseniy Yatsenyuk told an urgent government meeting in Kiev that his ex-Soviet country's conflict with its giant nuclear-armed neighbour was threatening to spiral out of control.

"The conflict is shifting from a political to a military stage," Yatsenyuk said in remarks broadcast live across the culturally splintered nation of 46 million people.

"Russian soldiers have started shooting at Ukrainian military servicemen, and that is a war crime," Yatsenyuk said.

Ukraine's interim president Oleksandr Turchnynov later issued a statement placing responsiblity for "the blood of Ukrainian soldiers (on) the leadership of the Russian Federation and specifically President Putin."

Regional defence ministry spokesman Vladislav Seleznyov told AFP the Ukrainian soldier had died after being shot in the neck when a group of gunmen stormed a military base in the northeast of Crimea's main city of Simferopol.

Seleznyov said another soldier was wounded but did not specify whether the base was stormed by Russian soldiers or pro-Kremlin militia who also patrol the peninsula.

But the Ukrainian defence ministry said in a statement the military base was attacked by people "dressed in the military uniforms of servicemen of the armed forces of the Russian Federation." (...)