Ukraine's acting president is to hold talks with law enforcement agencies to discuss "the dangerous signs of separatism" following the departure of Viktor Yanukovich.
Oleksander Turchinov, speaker of the assembly and Ukrainian acting president, made the comments during a parliamentary session on Tuesday but gave no further details. Some Russian-speaking parts of the country oppose Yanukovich's overthrow and attempts to bring the country closer to Europe, Reuters news agency reported.
Also on Tuesday, Ukraine's parliament put off plans to vote on the formation of a national unity government until Thursday to allow consultations to continue. The vote had been expected to take place during Tuesday's session as that was the deadline Turchinov had given at the weekend.
Politicians have been trying to stabilise the country after the disappearance of Yanukovich and months of violence.
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A presidential election campaign has already started; the Ukrainian Central Election Commission posted an election calendar online early on Tuesday, giving candidates until April 4 to register. Elections are due to be held on May 25.
Yanukovich’s whereabouts remain unknown after he left the capital Kiev at the weekend. Ukrainian authorities were issuing an arrest warrant for him over his alleged role in the killing of protesters by security forces.
In Washington, the White House indicated it no longer recognised Yanukovich as president.
Spokesman Jay Carney said although Yanukovich “was a democratically elected leader, his actions have undermined his legitimacy and he is not actively leading the country at present”.
Jen Psaki, a US State Department spokeswoman, said in a press briefing on Monday: “Yanukovich left Kiev. He took his furniture, packed his bags, and we don’t have more information on his whereabouts. So there are officials who have stepped in and are acting in response to that leadership gap at the moment.”
But Russian Foreign Minister Sergei Lavrov warned on Tuesday that it was "dangerous" to force Ukraine to choose between Russia and the west, a reference to interventions from the US and Europe over the fate of the Eastern European country.
He said: "It is dangerous and counterproductive to try to force upon a Ukraine a choice on the principle: 'You are either with us or against us'," Lavrov made the remarks at a joint news conference in Moscow following talks with Luxembourg Foreign Minister Jean Asselborn, news agency Reuters reported.
Turchinov has already said his country is ready for talks with Russia to try to improve relations, but has made it clear that Kiev's European integration is a priority.