"If there is an annexation of Crimea, if there's a referendum that moves Crimea from Ukraine to Russia, we won't recognise it, nor will most of the world," White House Deputy National Security Advisor Tony Blinken told CNN in an interview.
Crimea's regional pro-Moscow government will hold a referendum on joining Russia on March 16, which could lead to a formal annexation of the territory.
Blinkan said the US and its allies would increase pressure on Russia if it made the wrong choice.
"And so if Russia makes the wrong choice going forward, we have the ability to exert significant pressure on Russia, as do our partners," he said.
Blinken said Russia Russia is paying a heavy price for its military action in Ukraine.
"In coordination with our allies, we've imposed significant sanctions on Russia, and that's already exerting a cost. We've seen Russian markets go down substantially, the ruble go down, and investors sitting on the fence," he said.
US President Barack Obama, he said, has mobilised the international community in support of Ukraine to isolate Russia for its actions in Ukraine and to reassure its allies and partners, and that is having a clear effect.
Blinken said the US wants to resolve this diplomatically.
"I think the door is clearly open to resolving this diplomatically. Secretary (of State, John) Kerry is engaged with his Russian counterpart, Foreign Minister (Sergei) Lavrov. European leaders are also engaged with (the Russian President, Vladimir) Putin, as is the (US) President," he said.
