People cast their vote at a polling station on March 16, 2014 in Simferopol
The United States on Sunday strongly rejected a vote in Crimea on breaking away from Ukraine, and called Russian actions in the crisis "dangerous and destabilizing."
"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 spokesman Jay Carney said.
The White House statement came as Crimea's pro-Moscow authorities announced that exit polls showed a 93 percent vote in favor of becoming part of Russia.
"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.
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 destabilizing," the White House spokesman said.
"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 destabilizing actions," he said.
The White House condemnation followed further talks between US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov earlier Sunday.
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's overture appeared to receive a positive echo in Moscow, where the foreign ministry said Lavrov and Kerry agreed to look for ways to defuse the crisis through "the launch as soon as possible of a constitutional reform with the support of the international community."