U.S. President Barack Obama told his Russian counterpart Vladimir Putin Sunday that Washington and its "European partners are prepared to impose additional costs" on Moscow for backing a secession referendum in Ukraine's Crimean peninsula.
A White House statement called Sunday's referendum illegal and said it violates Ukraine's constitution. It also said the vote will "never be recognized by the United States and the international community."
News of the phone call came as Crimean election authorities said 95 percent of voters cast ballots supporting secession and a move to join Russia. The data was announced late Sunday with half of the referendum ballots counted. Voter turnout was placed at between 75 and 80 percent.
Crimea's pro-Moscow leader, Sergei Aksyonov, announced that his government will formally apply on Monday to join the Russian Federation.
In Kyiv, Ukraine's interim Prime Minister Arseniy Yatsenyuk -- speaking at an emergency cabinet meeting -- called the Moscow-backed Crimea vote "a circus spectacle" directed at gunpoint by Russia.
An earlier White House statement said no decision should be made about the future of Ukraine without the Ukrainian national government.
It also said the presidential elections planned for May 25 will provide a legitimate opportunity for all Ukrainians to make their voices heard on the future of their country.
Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on Sunday agreed to push for Ukrainian constitutional reforms for power sharing and decentralization as a solution to the crisis.
In Kyiv, Ukraine's acting defense minister told reporters that both Ukraine and Russia have agreed on a truce in Crimea until March 21.
Sunday's vote came a day after Russian forces seized a natural gas facility just outside Crimean territory. The Ukrainian Foreign Ministry called the move "a military invasion by Russia."
Ukraine provides the peninsula with all of its water and energy needs, and some analysts say the seizure may be aimed at ensuring the peninsula's energy requirements are met in the event Kyiv were to cut off supplies.