Washington - The US strongly rejected Crimea's vote on Sunday on breaking away from Ukraine, and called Russian actions in the crisis "dangerous and destabilising".
Crimea's pro-Moscow authorities announced that exit polls showed a 93% vote in favour of becoming part of Russia.
"This referendum is contrary to Ukraine's constitution, and the international community will not recognize the results of a poll administered under threats of violence and intimidation from a Russian military intervention that violates international law," White House spokesperson 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 outreach to Ukraine and calls for international monitoring, instead escalating its military intervention into Crimea and initiating military exercises on Ukraine's eastern border.
"Russia's actions are dangerous and destabilising," the Carney 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."Imminent action?
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.
Last week, the United States imposed visa bans targeting Russians and Ukrainians blamed for threatening the sovereignty or territorial integrity of Ukraine.
President Barack Obama also signed an executive order paving the way for economic sanctions against individuals or entities in Russia.
The order sets broad criteria, and could target those accused of usurping peace in Ukraine or Russian officials seeking to impose control over any part of the country.
US sanctions are expected to be mirrored by several Western powers, with the European Union readying a package of measures including visa bans and asset freezes.
The EU decried the Crimean referendum on as "illegal and illegitimate" saying its outcome "will not be recognised".
However, diplomats in Brussels say the EU sanctions will not target members of the Russian government so long as the West is attempting to negotiate a solution to the crisis with Moscow.