Biden vows more sanctions on Russia
US vice president Joe Biden says the world has seen through Russia's actions in Crimea and will impose more sanctions on it.
After meeting in Warsaw with Polish prime minister Donald Tusk, he said the US joins Poland and the international community in condemning the continuing assault on Ukraine's sovereignty.
Mr Biden said it was a blatant violation of international law.
He said virtually the entire world rejects the referendum in Crimea that cleared the way for Russia to annex the peninsula in Ukraine.
The comments come just after Russian president Vladimir Putin signed a treaty to annex Crimea.
Denouncing Russia's actions as a "land grab", Mr Biden warned Russia the US and Europe will impose further sanctions.
"The world has seen through Russia's actions and has rejected the flawed logic," he said.
Russia's move in clear defiance of its neighbours and the US ups the pressure on Mr Biden to convince its Nato allies that the US will not stand idly by.
His visit to the region is part of a broader US campaign to send a clear signal to Mr Putin following Sunday's referendum in Crimea, which the US has dismissed as illegal.
In coordination with Europe, the administration of President Barack Obama has frozen the US assets of nearly a dozen Russian and Ukrainian officials.
But Putin appears to have reacted with a shrug, and Mr Obama's critics contend the US steps thus far amount to a slap on the wrist.
The three Baltic nations are deeply concerned about what Russia's military intervention in Ukraine's Crimean Peninsula might portend for the region. All four countries share borders with Russia; Poland also borders Ukraine.