Russia's upper house of parliament has approved President Putin's request for Russian forces to be used in Ukraine.
He had asked that Russian forces be used "until the normalisation of the political situation in the country".
Russia's Black Sea Fleet is based in the Ukrainian region of Crimea, where many ethnic Russians live, BBC reports.
Kiev has reacted angrily to days of military movements in Crimea, accusing Moscow of trying to provoke the new government into an armed conflict.
Interim President Oleksander Turchynov has called an emergency session of his security chiefs.
Russia's Vladimir Putin submitted the request for troops "in connection with the extraordinary situation in Ukraine and the threat to the lives of Russian citizens", the Kremlin said.
The BBC's Richard Galpin in Moscow notes that he refers to the "territory of Ukraine" rather than specifically to Crimea, implying that troops could be used in other pro-Russian parts of eastern and southern Ukraine.
The request was approved unanimously within hours of it being made. During the debate, on legislator accused US President Barack Obama of crossing "a red line" with his comments on Friday that there would be costs if Russia intervened militarily in Ukraine.
The upper house has recommended that the Russian ambassador the US should be recalled, although the decision lies with Mr Putin.
