* US vice president visiting Poland and Lithuania
* Biden: NATO security commitment is unwavering
* East Europeans anxious after Russian action in Crimea
* Biden to discuss NATO support, energy security (Adds comments from briefing with senior administration official)
By Roberta Rampton
WARSAW, March 18 (Reuters) - The United States may run more ground and naval military exercises to help Baltic states near Russia beef up their capacity, Vice President Joe Biden said on Tuesday to reassure NATO allies alarmed by the Crimean crisis.
Moscow's despatch of troops to Ukraine's Crimean 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.
"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," he said after talks with Estonian President Toomas Hendrik Ilves, who was visiting Warsaw.
A senior administration official told reporters more details about the proposed new military exercises would be released in the days ahead. "This would not be a fundamental expansion, or crossing of a basic line, so to speak," said the official, speaking on condition of anonymity.
"It's more an opportunity to enhance our capacity to do training with them actually in the region."
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.
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 defence 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-defence 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 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.
Biden also said the United States wants to help eastern European states find ways to reduce their dependence on imported Russian fuel, a relationship that, U.S. officials say, the Kremlin uses as a tool of political influence.
The U.S. government could offer technical and regulatory assistance to countries like Poland that seek to tap their own supplies of shale gas, the senior administration official told reporters, noting Poland's Tusk recently proposed new streamline rules for drilling.
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.
But because of the lead time required to approve export licenses and build facilities, increased U.S. exports are more of a long-term consideration, the official said. (Additional reporting by Karolina Slowikowska and Pawel Bernat; Writing by Christian Lowe; Editing by Tom Heneghan)