The Crimean parliament has voted to leave Ukraine and join Russia, with the region's deputy prime minister saying the decree was effective immediately and that Russian soldiers were the only legitimate forces in Crimea.
The parliament on Thursday unanimously adopted a motion for the strategic peninsula to join the Russian Federation.
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Crimea's deputy prime minister, Rustam Temurgaliyev told Reuters: "The Ukrainian armed forces have to choose: lay down their weapons, ... accept Russian citizenship and join the Russian military."
"If they do not agree, we are prepared to offer them safe passage ... to their Ukrainian homeland."
Ukraine's prime minister, Arseniy Yatsenyuk, said the vote was "illegitimate", while the acting president, Oleksander Turchinov, said the vote dictated by fear by MPs working "under the barrel of a gun".
The Crimea parliament also said a referendum on the region's status was being brought forward from March 30 to March 16. Temirgaliev said there would be two questions on the ballot, the first asking whether Crimea should be part of Russia.
However, Al Jazeera's Hoda Hamid, reporting from Sevastopol, said there were serious questions about the legitimacy of the parliament, the prime minister, and the decrees.
"The prime minister came to power arguably at gunpoint when the parliament was taken over," she said, referring to a takeover of the building by pro-Russian forces last week.
"The constitution also says parliament cannot take such a decision," she said, adding that Russia would also have to formally accept the decree.
About 11,000 pro-Russian troops are in control of 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 those troops are believed to be Russian, even though Moscow has repeatedly denied sending them.
Some of those troops on Thursday blocked military observers from the Organisation for Security and Co-operation
in Europe from entering Crimea, the group said,
The US president, Barack Obama, meanwhile issued an executive order on Thursday saying that Russia's involvement in Crimea constituted "an unusual and extraordinary threat to the national security and foreign policy of the United States".
His order authorised sanctions against "individuals and entities responsible for activities undermining democratic processes or institutions in Ukraine.''
The EU meanwhile announced that it was to suspending talks with Russia on a wide-ranging economic pact and on a visa deal. They also threatened further sanctions if Russia does not quickly engage in talks to end the crisis.
After a meeting of EU ministers in Brussels, Donald Tusk, Poland's prime minister, said the EU members of G8 had also agreed to suspend preparations for a G8 summit in June in Sochi.
Diplomatic efforts to cool the crisis continued on Thursday. The US secretary of state, John Kerry, met his Russian counterpart Sergey Lavrov on the sidelines of a meeting in Rome, but little appeared to have been resolved.
"For the moment we cannot report to the international community that we are in agreement," Lavrov was quoted by Russian news agencies as saying.
"We want to clarify more deeply what our partners mean when they suggest some kind of international mechanism, what its content would be," he added.
Kerry "made clear the importance of the Russians talking directly to the Ukrainians", US diplomatic sources told journalists after the 40-minute meeting.
The US also announced that the guided-missile destroyer, the USS Truxtun, was heading to the Black Sea area, in a deployment planned before the Ukraine crisis began.
According to reports in Poland, the US was also sending a squadron of F-16 fighter jets there for a training exercise.