MPs in the southern Ukrainian region of Crimea have voted to formally become part of the Russian Federation.
Parliament said the decision would be put to the Crimean people for their verdict in a referendum on 16 March.
A government minister in Kiev said they believe it would be unconstitutional for Crimea to join Russia.
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, which already enjoys a degree of autonomy from Kiev, for several days.
The announcement from Crimea’s parliament comes as EU leaders are meeting in Brussels to discuss how to respond to Russia’s troop deployment on Ukrainian soil.
The Crimean parliament resolved “to enter into the Russian Federation with the rights of a subject of the Russian Federation”.
Kremlin spokesman Dmitry Peskov told Russian media that President Vladimir Putin had been informed of the Crimean vote.
Sources in the Crimean parliament have told the BBC they are now waiting for a response from Moscow to their request for Crimea to become a part of Russia.
According to the decree published on the Crimean parliament’s website, citizens will be asked in the referendum whether they are in favour of reuniting the region with Russia “as a subject of the Russian Federation” or “retaining the status of Crimea as part of Ukraine”.
Ukraine’s interim Economy Minister Pavlo Sheremeta, speaking in Kiev soon after the announcement was made, said: “We’re not working out what to do if Crimea joins the Russian Federation because we believe it’s unconstitutional.”
The move by Crimea’s parliament will significantly increase tensions as Western diplomats try to draw political leaders in Ukraine and Russia into negotiations to prevent a full Russian invasion of Ukraine, the BBC’s Richard Galpin reports from Moscow.