The United States has strongly rejected Crimea's vote on breaking away from Ukraine and called Russian actions in the crisis "dangerous and destabilising".
Crimea's pro-Moscow authorities say exit polls show a 93 per cent vote in favour of becoming part of Russia.
"This referendum is contrary to Ukraine's constitution, and the international community will not recognise the results of a poll administered under threats of violence and intimidation from a Russian military intervention that violates international law," White House spokesman Jay Carney said.
"The United States has steadfastly supported the independence, sovereignty and territorial integrity of Ukraine since it declared its independence in 1991, and we reject the 'referendum' that took place today in the Crimean region of Ukraine."
Carney said Russia had spurned calls for international monitoring, instead escalating its military intervention into Crimea and initiating military exercises on Ukraine's eastern border.
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"Russia's actions are dangerous and destabilising," the spokesman added.
"As the United States and our allies have made clear, military intervention and violation of international law will bring increasing costs for Russia - not only due to measures imposed by the United States and our allies but also as a direct result of Russia's own destabilising actions."
The White House condemnation followed another round of talks earlier between US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov.
In a phone call with Lavrov, Kerry urged Moscow to pull back Russian forces to their bases in Crimea in exchange for constitutional reforms to protect minority rights.
A senior State Department official said Kerry "made clear that this crisis can only be resolved politically and that as Ukrainians take the necessary political measures going forward, Russia must reciprocate by pulling forces back to base, and addressing the tensions and concerns about military engagement".
Kerry also raised concerns about Russian military activity in Kherson Oblast, the Ukrainian province just north of Crimea, and "continuing provocations" in the eastern cities in Ukraine, the official said.
Kerry had on Friday already warned of sanctions against Russia if the referendum went ahead, making it likely that some form of action is now imminent.
