US 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 US 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 US 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 recognise 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 US stood by its commitment to complete a missile defence system in Poland by 2018.

Meanwhile, the US and its G7 allies will gather next week at The Hague to consider a further response to Russia's attempt to absorb Ukraine's Crimea region, the White House said.

The G7 meeting will take place on the margins of a nuclear security summit at The Hague that US President Barack Obama plans to attend.

"The meeting will focus on the situation in Ukraine and further steps that the G7 may take to respond to developments and to support Ukraine," said White House National Security Council spokeswoman Caitlin Hayden.

The G7 includes the US, Germany, Britain, Canada, France, Japan and Italy. The European Union was also invited to the talks.

The G7 leaders already have suspended preparations for a G8 summit, which includes Russia, that is scheduled for June in Sochi, Russia. Russia became the eighth member of the group in 1998.  -Reuters