The White House on Monday said President Barack Obama would speak to Russian President Vladimir Putin soon, perhaps later in the day, and made clear the United States was not considering lethal aid for Ukraine.

"We are looking at a variety of ways to demonstrate our strong support for Ukraine including diplomatically and economically," White House spokesman Jay Carney told reporters.

"We're not actively considering lethal aid but we are reviewing the kinds of assistance we can provide," he said.

This after an adviser to Secretary of State John Kerry said earlier on Monday that the United States is considering supplying arms to Ukraine, where unrest in eastern cities bears the hallmarks of a Russian destabilization drive.

Both statements came as the standoff in eastern Ukraine grew increasingly hostile Monday morning as the country's acting president vowed to forcibly clear out pro-Russian insurgents occupying government buildings. Meanwhile, Russia's foreign minister warned Western powers against encouraging such a move, saying that it's not in Russia's interest to see Ukraine falling apart.

The mutual threats came as insurgents in Slaviansk, Donetsk and other eastern cities ignored the 6 GMT (9 A.M. Israel time) deadline declared Sunday night by President Oleksander Turchinov, who demanded they vacate the government buildings they had seized and hand over their weapons.

As the deadline passed, Turchinov said the Ukrainian army would begin an anti-terrorist operation against the separatists, vowing that the eastern Ukrainian region they claimed "will soon be stabilized." At the same time, Russian Foreign Minister Sergei Lavrov said any powers that encourage Kiev to use force against protesters must take full responsibility for their actions.

Turchinov said the Kiev leadership was "not against" a referendum being held on the type of state Ukraine should be and added he was sure it would confirm the wish of the majority for a united, independent country.

The holding of a referendum has been one of the demands of separatist rebels.

Lavrov backed the idea of a referendum on a constitution for Ukraine, adding that it should be drafted with the involvement of the country's Russian-speaking eastern regions.

Speaking at a news conference after meeting his Sudanese counterpart, Lavrov said it was not in Russia's interests for Ukraine to break up, but that Moscow wanted all citizens of the country to be given equal treatment by Kiev.

He denied Ukrainian and U.S. allegations that Russia had undercover agents fomenting unrest in eastern Ukraine, and said he was seeking explanation of media reports that the director of the CIA, John Brennan, had visited Kiev.

Turchinov and other leaders blame Russia, which annexed Ukraine's Crimea region when Moscow-backed former president Viktor Yanukovich fled after months of pro-Western protests, for inspiring and organizing a rash of rebellions in Slaviansk and other Russian-speaking towns in eastern Ukraine.

"We will not allow Russia to repeat the Crimean scenario in the eastern regions of Ukraine," Turchinov said on Sunday night.

The crisis has brought relations between Russia and the West to their lowest point since the end of the Cold War in 1991, and also carries a risk of unleashing a "gas war" which could disrupt energy supplies across Europe.

Use of force by Kiev's pro-Europe authorities could trigger a fresh confrontation from Russia. Russia's foreign ministry called the planned military operation a "criminal order" and said the West should bring its allies in Ukraine's government under control.

The United Nations Security Council held an emergency session on Sunday night, and the United States warned that it was likely to impose further sanctions on the Kremlin if the escalation in eastern Ukraine continues.