"We condemn Russia's moves to formally annex the Crimean region of Ukraine. Such action is a threat to international peace and security and it is against international law," White House Press Secretary Jay Carney said.
"We would not recognise this attempted annexation. As we have said, there are costs with such action," he told reporters at his daily news conference.
Russian President Vladimir Putin today signed a treaty adding Crimea to the map of Russia, in a move that would mark the most significant redrawing of Europe's borders since World War II.
The Obama administration is currently engaged in de-escalation of tension in the region, he said as he noted that the US is in full support of Ukraine in its hour of crisis.
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"To our NATO allies, our message is clear: We have a solemn commitment to our collective defence and we will uphold this commitment," he said.
US President Barack Obama has invited his counterparts from Canada, France, Germany, Italy, Japan, the UK and the European Union to a meeting of G-7 leaders next week on the margins of the nuclear security summit in the Hague, he said.
"Those actions have incurred costs already. They have done damage to Russia's economy, to its currency and to its standing in the world. Further actions, further provocations will lead to higher costs," he warned.