The Russian parliament granted President Vladimir Putin permission to use the country's military in Ukraine.
According to AP, Saturday's unanimous vote formalized what Ukrainian officials described as an ongoing deployment of Russian troops in the strategic region of Crimea. It also raised the possibility that Moscow could send its military elsewhere in Ukraine.
Putin's call came as pro-Russian demonstrations broke out in Ukraine's Russian-speaking east, where protesters raised Russian flags.
Dozens were hurt when a pro-Russia protest in Ukraine's eastern city of Kharkiv turned violent, with demonstrators trying to storm the local government building, an AFP reporter said.
Some 20,000 joined the protest against Kiev's new pro-West government, and later around 300 launched the assault on the government building. Stones and stun grenades were thrown though it was unclear by whom.
Russia's move sharply raised the stakes in the conflict following the ouster of Ukraine's President Viktor Yanokovych last week by a protest movement aimed at turning Ukraine toward the European Union and away from Russia.
Obama warns about costs
U.S. President Barack Obama warned Moscow on Friday ''there will be costs'' if Russia intervenes militarily.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," he told reporters.
Obama and European leaders would consider skipping a G8 summit this summer in the Russian city of Sochi if Moscow intervenes militarily in Ukraine, a senior U.S. official said, according to Reuters.
Germany described developments in the last few hours as dangerous. British foreign minister William Hague said he had spoken to Russian counterpart Sergei Lavrov and called for a 'de-escalation' of tensions, Reuters reported.
Russia's parliament, flying in the face of Obama's warning against any Russian intervention, urged President Vladimir Putin to "to take measures to stabilize the situation in Crimea and use all available means to protect the people of Crimea from tyranny and violence."
Earlier in the day, Kremlin said that Putin asked parliament for permission to use the country's military in Ukraine.
Putin said the move is needed to protect ethnic Russians and the personnel of a Russian military base in Ukraine's strategic region of Crimea.
"I'm submitting a request for using the armed forces of the Russian Federation on the territory of Ukraine pending the normalization of the socio-political situation in that country," Putin said in a statement released by the Kremlin.