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A referendum had previously been scheduled in Crimea on March 30, but the question to be put to voters was on whether their region should enjoy "state autonomy" within Ukraine.

The region's deputy prime minister, Rustam Temirgaliev, said there would now be two questions on the ballot. "The first one: Are you in favor of Crimea becoming a constituent territory of the Russian Federation? The second one: Are you in favor of restoring Crimea’s 1992 constitution?"

Crimea was given to Ukraine, then part of the Soviet Union, by Soviet leader Nikita Khrushchev in 1954. When the U.S.S.R. collapsed, the peninsula officially became part of the newly formed state of Ukraine, although Russia has maintained its strategic Black Sea Fleet base there.

Temirgaliev added that Crimea could adopt the Russian currency, the ruble, and nationalize state property. "We are ready to introduce the ruble zone," he said, as quoted by Interfax news agency.

Ukraine's interim president, Oleksander Turchinov, responded curtly to the vote by saying that Crimean authorities are “totally illegitimate” and motivated by fear. “They are forced to work under the barrel of a gun, and all their decisions are dictated by fear and are illegal,” his spokeswoman quoted him as saying.

President Barack Obama echoed Kiev’s condemnation in comments to reporters gathered at the White House, calling the secession vote a “violation of international law.”

“Any discussion about the future of Ukraine must involve the government of Ukraine,” Obama said.

Later Thursday, Obama spoke with Russian President Vladimir Putin by phone.

According to a White House readout of the call, while reaffirming the U.S. opposition to Russia’s “violation of Ukraine’s sovereignty and territorial integrity,” Obama urged Putin to facilitate a diplomatic solution between the two powers.

According to Obama, solving the current standoff would entail international support for direct talks between Russia and Ukraine, monitoring the rights of all Ukrainians, including its ethnic Russian population, removal of Russian troops from Ukrainian land and preparation for Ukrainian elections in May.

About 11,000 pro-Russian troops are in control of all access to the peninsula and have blocked all Ukrainian military bases that have not yet surrendered, according to the regional leader, Sergei Aksyonov. All or most of these troops are believed to be Russian, although Moscow has repeatedly denied sending in soldiers, and they are not wearing any insignia.