More than 95 percent of voters in Ukraine's Crimea region supported union with Russia in a referendum, according to partial results quoted by Russian state news agency RIA.
It said the figure was provided by Mikhail Malyshev, head of the referendum commission, after more than half the votes were counted on Sunday, on the Black Sea peninsula where Russian forces have seized control.
Another Russian news agency, Interfax, said voter turnout had exceeded 80 percent.
Al Jazeera's Peter Sharp, reporting from Moscow, said that "most people in Russia are delighted by the news", with reports that Russian President Vladimir Putin is "enjoying his highest popular rating in three years, as it jumped ten percent in the last six weeks since the move on Crimea".
The results of vote show that the "diplomacy going on for many weeks now have proven to be a failure", Sharp said, adding that an announcement of sanctions imposed by the US and EU is expected on Monday.
US President Barack Obama told Putin in a phone call that the United States rejected the results of the referendum and warned that Washington was ready to impose sanctions on Moscow over the crisis.
"He (Obama) emphasised 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," the White House said in a statement.
Obama told Putin the crisis could still be resolved diplomatically, but said the Russian military would need to first stop its "incursions" into Ukraine, the White House said.
Arseniy Yatsenyuk, Ukraine's new prime minister, echoed the sentiments, and said that neither Kiev nor the West would recognise the referendum's results, which he said had been conducted at gunpoint.
"Now, on the territory of the autonomous republic of Crimea under the staged direction of the Russian Federation, a circus performance is underway: the so-called referendum," Yatsenyuk told a government meeting.
"Also taking part in the performance are 21,000 Russian troops, who with their guns are trying to prove the legality of the referendum."
|Kiev denounces Crimea referendum
In a phone call with German Chancellor Angela Merkel, Putin promised to respect Crimeans' decision, and said that the referendum complied with international law.
There were two options available in the referendum. Firstly, "are you in favour of the reunification of Crimea with Russia as a part of the Russian Federation?"
The second option was "are you in favour of restoring the 1992 Constitution and the status of Crimea as a part of Ukraine?"
Returning to the 1992 constitution would give Crimea's government sweeping powers to make its own laws and control its own governance, while technically remaining part of Ukraine.
Pro-Russian armed forces took control of the strategic peninsula with a majority ethnic-Russian population soon after the Kremlin-backed Ukrainian President Viktor Yanukovich fled Kiev last month after three months of deadly protests against his rule.