MPs in Crimea have asked Moscow to allow the southern Ukrainian region to become part of the Russian Federation.
Parliament said if its request was granted, Crimean citizens could give their view in a referendum on 16 March.
Ukraine's interim Prime Minister Arseniy Yatsenyuk said the move had no legal grounds.
Crimea, a region whose population is mostly ethnic Russian, has been at the centre of tensions following the fall of Ukraine's pro-Moscow president.
Pro-Russian and Russian forces have been in de facto control of the peninsula for several days.
The announcement from Crimea's parliament comes as EU leaders meet in Brussels to discuss how to respond to Russia's troop deployment on Ukrainian soil.
Meanwhile, Washington says it is issuing visa restrictions on a number of Ukrainian and Russian officials and individuals in line with a policy "to deny visas to those responsible for, or complicit in, threatening the sovereignty and territorial integrity of Ukraine".
The Crimean parliament resolved "to enter into the Russian Federation with the rights of a subject of the Russian Federation".
It said it had asked Russian President Vladimir Putin "to start the procedure".
"This means we have reunited with our motherland which we have been a part of for so long," said Crimea's deputy parliamentary speaker, Sergei Tsekov.
The Kremlin said President Putin was aware of developments but no response has yet been made.
If Russia agrees to Crimea's request, the Crimean people will be asked two questions in the 16 March referendum, the statement says:
Mr Tsekov said he believed most Crimeans would "support our decision at the referendum".
Ukraine's new interim government does not recognise the leadership in Crimea – which was sworn in at an emergency session while the building was under siege from pro-Russian armed men last week.
Interim Economy Minister Pavlo Sheremeta said it would be unconstitutional for Crimea to join the Russian Federation.
But Crimea's Deputy PM Rustam Temirgaliev dismissed the suggestion, saying Crimea views the new authorities in Kiev as illegitimate.
Mr Yatsenyuk met the 28 EU leaders before their emergency meeting in Brussels.
He said that Ukraine had to be "one united" nation and that Ukrainians stood ready to "protect our country" if Russia extended its military action in the country.
He noted there was a "huge military imbalance", saying: "We have less arms, no nuclear weapons… but we have spirit."
He added: "Russian boots and tanks on the ground is unacceptable in the 21st Century."
Some EU members have been calling for tough sanctions on Russia, while others – led by Germany – prefer mediation.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov met in Rome on the sidelines of a conference on Libya.
After the talks, Mr Lavrov said: "For now we cannot tell the international community that we have an agreement."
In other developments:
Pro-Russian gunmen moved in to seize strategic sites in Crimea after Viktor Yanukovych was ousted as the president of Ukraine following months of protests in Kiev.
The demonstrations – by Ukrainians seeking closer ties with the West – turned violent in mid-February with more than 90 people killed in clashes with police.