Russian President Vladimir Putin has signed a decree recognising Crimea as a “sovereign and independent state”, officials say.
Sources said the decree would come into force immediately.
The move follows Sunday’s referendum in Crimea in which officials said 97% of voters backed breaking away from Ukraine and joining Russia.
The EU and US earlier imposed sanctions on a number of officials from Russia and Ukraine over the vote.
The Russian presidential decree was issued “considering the expression of the will of the people of Crimea at the general Crimean referendum, which was held on 16 March 2014,” the text of the decree said.
Earlier, the Crimean parliament declared independence from Ukraine and applied to join Russia.
More sanctions threatened
The referendum was called by the pro-Russian authorities in Crimea after the ousting of Ukraine’s pro-Moscow President Viktor Yanukovych, who had sparked months of street protests by rejecting a planned EU trade deal in favour of closer ties with Moscow.
Pro-Russian forces have been in control of Crimea since late February. Moscow says the troops are self-defence forces and not under its direct control.
The EU, US and authorities in Kiev have rejected the referendum as illegal.
The EU and US earlier published separate lists of sanctions against both Russian and Ukrainian government officials and MPs.
They included Crimea’s acting leader Sergei Aksyonov speaker of parliament Vladimir Konstantinov.
The US list included Dmitry Rogozin, a Russian deputy prime minister, Valentina Matviyenko, head of the upper house of the Russian parliament and Mr Yanukovych.
US President Barack Obama said in a news conference that Washington stood “ready to impose further sanctions” depending on whether Russia escalated or de-escalated the situation in Ukraine.
But he also stressed there was still a path to solve the crisis diplomatically.
‘Negative spiral’
EU foreign policy chief Catherine Ashton said those singled out for travel and asset bans were responsible for threatening Ukraine’s territorial integrity and independence.
But there was still time to avoid “a negative spiral” in the situation, she added, urging Russia to withdraw its forces from Crimea.
