Ukraine's interim president says he has launched a crackdown on pro-Russian demonstrators who have taken over state buildings in eastern Ukraine.
Oleksander Turchynov says government forces began a gradual and cautious operation in the Donetsk region, where pro-Moscow forces have ignored Kyiv's previous ultimatums to surrender.
Mr. Turchynov told lawmakers Moscow has intentionally inflamed the situation in the east.
to be on fire. They want the whole south and east of Ukraine to be on fire - from Kharkiv region to Odessa region.''
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Opposition leader Vitaly Klitschko urged Mr. Turchynov to send Ukrainian armed forces to the region to drive out the separatists.
Russian Foreign Minister Sergei Lavrov says Moscow will withdraw from an emergency international summit scheduled for Thursday in Geneva if Kyiv uses force in eastern Ukraine.
Lavrov was questioned about the situation Tuesday during a visit to China.
And Russian Prime Minister Dmitry Mevedev told reporters at a Moscow news conference Tuesday that he believes Ukraine is on the brink of civil war, a prospect he said is "scary."
Tensions over the crisis spilled into the streets of Kyiv on Tuesday when dozens of activists assaulted pro-Russian lawmaker Oleh Tsaryov outside a television studio. Tsaryov is a candidate in presidential elections to be held May 25.
Meanwhile, Russian state media say at least two people have been injured in the clashes near an airport at Kramatorsk, but those reports were not independently confirmed.
Western governments have accused Moscow of backing the protests in eastern Ukraine. U.S. President Barack Obama warned Russian President Vladimir Putin in a phone call Monday that Moscow will face further costs if its actions in Ukraine persist. He urged the Russian leader to use his influence to persuade the demonstrators to leave the buildings they have seized.
In turn, Mr. Putin urged Mr. Obama to discourage the Ukrainian government from using force against protesters.
The demonstrators are demanding a referendum on whether to split with Ukraine and join Russia - similar to the recent vote in Crimea.
A report issued Tuesday by the U.N. human rights commissioner accuses Russia of manipulating the March 16 vote through propaganda, harassment and torture of anti-Russian activists. It said ethnic Russians in eastern Ukraine have falsely claimed to be under assault, to justify Russian intervention. The report warned that if the propaganda persists, it will have an impact on Ukraine's May 25 presidential election.
White House officials say U.S. Vice President Joe Biden will travel to Kyiv on April 22.