By Roberta Rampton

WARSAW (Reuters) - The United States is considering sending troops for war games in the Baltic states bordering Russia, Vice President Joe Biden said on Tuesday in an effort to reassure NATO allies alarmed at the Kremlin's intervention in Ukraine.

Moscow's despatch of troops to Ukraine's Crimea Peninsula and its unilateral declaration that the area is now part of Russia have left NATO member states in eastern Europe worried that they could be next in line.

In the Polish capital on the first leg of a two-day trip to the region, Biden condemned Russia's actions in Crimea as a land grab, and he said NATO's commitment to protect any of its members from attack was unwavering.

He also said the United States would work on reducing the dependence of eastern European states on imported Russian fuel, a relationship that, U.S. officials say, the Kremlin uses as a tool of political influence.

Biden's suggestion of sending U.S. troops for military exercises in the Baltics, if it is implemented, would represent the strongest concrete evidence to date of Washington's resolve in backing up its NATO allies on Russia's doorstep.

"We are exploring a number of additional steps to increase the pace and scope of our military cooperation including rotating U.S. forces to the Baltic region to conduct ground and naval exercises and training missions," Biden told reporters after talks with Estonian President Toomas Hendrik Ilves, who was visiting Warsaw.

"LAND GRAB"

Earlier, at a briefing alongside Polish Prime Minister Donald Tusk, Biden described Russia's actions as an assault on Ukraine's sovereignty and a violation of international law.

"Russia has offered a variety of arguments to justify what is nothing more than a land grab," Biden said. "But the world has seen through Russia's actions and rejected the logic, the flawed logic, behind those actions."

Russia said it sent troops to Crimea to protect Russian residents, who it said were in danger in the unrest that followed the toppling of Ukraine's pro-Russian president. A referendum in Crimea on Sunday backed union with Russia, though the West called that vote a Moscow-orchestrated sham.

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Biden said the events in Crimea were a reminder to NATO members that they need to stand together. He said Washington would take additional steps to strengthen NATO.

In particular, he said, the United States stood by its commitment to complete a missile defense system in Poland by 2018. Polish officials view that system as a barometer of Washington's readiness to underwrite their security.

"Recent events remind us that the bedrock of our alliance remains collective self-defense as enshrined in Article 5 of the NATO treaty," Biden said. "We take it deadly serious and our commitment is absolutely unwavering and unshakeable."

Military exercises in the Baltics would build on measures already taken. The Pentagon increased the number of U.S. aircraft taking part in regular NATO air patrols over the Baltics, and it beefed up a previously planned training exercise with the Polish air force.

The three Baltic states, Estonia, Latvia and Lithuania, are in NATO and the European Union. Yet they are particularly vulnerable to any Russian action. They are small, they depend on Russia for energy and trade, and they have sizeable Russian-speaking minorities.

From Warsaw, Biden is flying to Lithuania's capital, Vilnius, where on Wednesday he will meet Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins.

ENERGY SECURITY

U.S. officials said that as well as reaffirming the NATO security guarantees, Biden is bringing an offer of help in reducing reliance on Russian energy supplies.

"They'll discuss energy security and, including in that, long-term diversification of energy supply, so that energy can't be used as a political tool," a senior administration official told reporters travelling with Biden. The official cited shale gas and nuclear power as two areas for discussion.

The United States is poised to become a major exporter of liquefied natural gas in coming years, creating the potential for U.S. gas to at least partially displace Russian supplies.

Natural gas importers from around the world have urged the White House to speed up approvals of more export facilities so the fuel can be shipped to countries which now rely on Russian gas.

(Additional reporting by Karolina Slowikowska and Pawel Bernat; Writing by Christian Lowe; Editing by Alistair Lyon)