* US vice-president visiting Poland, Lithuania
* Biden: NATO security commitment is unwavering
* East Europeans anxious after Russian action in Crimea
* Biden to discuss NATO support, energy security (Updates with Biden comments)
By Roberta Rampton
WARSAW, March 18 (Reuters) - U.S. Vice President Joe Biden told NATO allies in eastern Europe on Tuesday that the alliance was unwavering in its commitment to protect them from attack, offering reassurance after what he called Russia's "land grab" in Ukraine's Crimea peninsula.
Biden was in the Polish capital at the start of a two-day mission to demonstrate to countries on Russia's western borders, nervous they could be next in line after Crimea, that the United States stands by them.
After talks with Polish Prime Minister Donald Tusk, Biden said they had discussed ways to reduce the region's dependence on imported Russian gas, which U.S. officials say the Kremlin uses as a lever of political pressure, and reviewed their mutual commitments as members of the NATO military alliance.
Biden spoke moments after Russian President Vladimir Putin, at a ceremony in Moscow, signed a document on Crimea's admission into the Russian Federation, although Ukraine's government and Western powers have said they will not recognize the move.
Putin acted after a referendum vote in Crimea, a majority ethnic Russian region of Ukraine, in favour of union with Russia following mass unrest that toppled Kiev's pro-Russian president. The West called the referendum a Moscow-orchestrated sham.
Russia's leaders had "rushed an illegal referendum in Crimea that was not surprisingly rejected by virtually the entire world", Biden, standing alongside Tusk, told a news conference.
"We join Poland and the international community in condemning the continuing assault on Ukraine's sovereignty and territorial integrity and the blatant violation of international law," he said.
"Russia has offered a variety of arguments to justify what is nothing more than a land grab But the world has seen through Russia's actions and rejected the logic, the flawed logic, behind those actions."
Biden said that the events in Crimea were a reminder to NATO members that they need to stand together. He said collective security guarantees were fundamental to the alliance and that Washington will take additional steps to strengthen NATO.
He said, in particular, that the United States stood by its commitment to complete a missile defence system in Poland by 2018.
"Recent events remind us that the bedrock of our alliance remains collective self defense as enshrined in Article 5 of the NATO treaty," he said. "We take it deadly serious and our commitment is absolutely unwavering and unshakeable."
The United States had previously offered reassurance to NATO allies in Western Europe since the start of the Crimea crisis. U.S. forces and equipment took part last week in war games in Poland and U.S. fighter jets patrolled over the Baltics.
Later on Tuesday Biden was to meet Polish President Bronislaw Komorowski and he will also hold talks with Estonian President Toomas Hendrik Ilves, who is in Warsaw on an official visit. On Wednesday in Vilnius he will meet Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins.
U.S. officials said that as well as reaffirming the NATO security guarantees, Biden will also be bringing an offer of help in reducing reliance on Russian energy suppliers.
"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 Obama administration to speed up approvals of additional export facilities so they can become less dependent on Russia.
Biden also wants to get an assessment from leaders about the impact of punitive sanctions imposed this week on Moscow by the United States and European Union, the senior administration official told Reuters.
As neighbours of Russia with close economic ties, retaliation for sanctions could be an issue. "He's going to hear an earful," said Heather Conley of the Center for Strategic and International Studies (CSIS) in Washington.
For Poland and the Baltics, which like Ukraine, were once under the Soviet Union's control, watching the Crimean crisis unfold was a "nightmare scenario," Conley said in an interview. (Additional reporting by Karolina Slowikowska; Writing by Christian Lowe and Roberta Rampton; Editing by Mark Heinrich)