WASHINGTON - President Barack Obama said on Sunday the United States will not recognize the Crimean referendum, vowing to impose fresh sanctions on Russia.

In his talks with Russian President Vladimir Putin over the phone, the American leader said the vote in the Ukraine's autonomous republic violated the Ukrainian constitution and was conducted "under duress of Russian military intervention," the White House said in a readout of the conversation.

The plebiscite, in which 95.5 percent of voters were in favor of rejoining Russia as shown by the preliminary results, "would never be recognized by the United States and the international community," Obama said.

He told Putin that "Russia's actions were in violation of Ukraine's sovereignty and territorial integrity and that, in coordination with our European partners, we are prepared to impose additional costs on Russia for its actions."

US Secretary of State John Kerry on Thursday threatened "a very serious series of steps" against Russia with the European Union as early as Monday depending on Russia's next moves.

Washington has canceled trade talks and military exchanges with Russia, and Obama has authorized visa bans and assets freeze on some Russians and Ukrainians blamed for Moscow's military takeover of Crimea.

Putin told Obama, however, that Crimea's vote on Sunday fully complied with the international law, according to the Kremlin's readout of the phone talks. Putin told German Chancellor Angela Merkel earlier in the day over the phone that Russia would "respect the choice" of the Crimeans.

Obama stressed that "there remains a clear path" for resolving the crisis diplomatically, in a way "that addresses the interests of both Russia and the people of Ukraine."

Washington and its allies have been pushing for Moscow to withdraw its military personnel to bases in Crimea, home to Russia's Black Sea Fleet, allow international monitors into Crimea and to open a dialogue with the Ukrainian government.

Obama reiterated that "a diplomatic resolution cannot be achieved while Russian military forces continue their incursions into Ukrainian territory and that the large-scale Russian military exercises on Ukraine's borders only exacerbate the tension," the White House said.

The two presidents agreed to continue to work together to find a diplomatic resolution, the Kremlin said.

Earlier on Sunday, Russian Foreign Minister Sergei Lavrov told U.S. Secretary of State John Kerry via a telephone that Moscow's position on Crimea referendum remained unchanged. He urged the US side to place its full influence on Kiev authorities to "stop mass lawlessness and arbitrariness against the Russian speaking population."

Crimea, historically part of the Russian Federation, was transferred to Ukraine in May 1954, then a republic of the Soviet Union.