The speaker of Russia's upper house of parliament says Russian lawmakers will support Crimea's decision if the Ukrainian region decides in a referendum to join Russia.
Ukraine's interim government, meanwhile, has rejected Crimea's moves to leave Ukraine, with Prime Minister Arseniy Yatsenyuk saying Friday that "no one in the civilized world" will recognize the results of the region's referendum.
Ukraine and Russia have been locked in a tense standoff since Russian forces entered the Crimean peninsula a week ago.
On Thursday, the Moscow-backed Crimean parliament voted to become part of Russia, setting a referendum for March 16.
The Ukrainian prime minister told reporters Friday he wants to "warn separatists" and others he described as "traitors of the Ukrainian state" that their decisions are "unlawful" and "unconstitutional." U.S. and European leaders have also called the referendum illegal.
U.S. President Barack Obama spoke by phone Thursday with Russian President Vladimir Putin, but the two leaders found little common ground. The White House says Mr. Obama told Mr. Putin the presence of Russian forces in Crimea is a violation of Ukraine's sovereignty. The Kremlin says Mr. Putin denounced Ukraine's new Western-backed government as "illegitimate" and said Russia cannot "ignore" calls for help from Ukraine's Russia-leaning east and south.
Ukrainian Prime Minister Yatsenyuk said Friday his government is "prepared to rebuild relations with Russia." But he said Russia must withdraw its troops, fulfill its agreements with Ukraine and stop supporting separatists in Crimea.
The White House says Mr. Obama, in his phone conversation with Mr. Putin, called for direct talks between Kyiv and Moscow that would be mediated by the international community. Mr. Obama called for all Russian forces to return to their bases and for international monitors to ensure the safety of Ukrainians, including ethnic Russians.
Pro-Western Ukrainian opposition icon Yulia Tymoshenko urged Europe Thursday to take strong action to prevent Ukraine's Crimean peninsula from joining Russia, saying such a move would destabilize the entire continent.
Witnesses and Western analysts say thousands of Russian military personnel have crossed into Crimea since last week. The reports set off a groundswell of Western condemnation against President Putin, and warnings of stiff penalties if Moscow fails to withdraw.
In Washington Thursday, President Obama authorized sanctions, including visa restrictions, against those found to have violated Ukraine's territorial integrity. The EU also imposed measures against Russia, suspending talks on visas and a new economic agreement.
On Capitol Hill, the U.S. House of Representatives voted to provide loan guarantees of $1 billion to Ukraine. That measure now goes to the U.S. Senate. The European Union is prepared to extend a $15 billion bailout to Kyiv if Ukraine can reach an agreement with the International Monetary Fund.
Ukraine's crisis began when protests erupted in late November after then-President Viktor Yanukovych rejected an economic deal with the EU in favor of closer ties with Russia. What began as peaceful protests quickly turned violent, leading to the deaths of more than 80 protesters and charges that the Yanukovych government ordered snipers to shoot protesters. Mr. Yanukovych fled Ukraine last month.