Vice President Joe Biden called the vote for Crimea to join Russia "illegitimate." He lashed out at Russia's support of the referendum as "a land grab" and a "blatant violation of international law."
"Russia stood alone, naked in front of the world," he said.
The veep vowed to slap Russia with more sanctions if they go through with the annexation. The EU, as well as the U.S, had already begun to approve some sanctions.
Biden also promised to continue supporting the government in Kiev.
"As we impose costs on Russia, we have to be equally resolute in supporting the government in Ukraine," he said.
Biden was speaking at a news conference in Warsaw, Poland after meeting Polish Prime Minister Donald Tusk.
"Russia's annexation of Crimea can't be accepted by the international community including Poland. In one moment this changes the country's (Ukraine) borders and the geopolitical situation in this region of the world," Tusk said, according to Reuters.
Watch part of Biden's speech above.