DONETSK, Ukraine (AP) — Pro-Moscow activists barricaded inside government buildings in eastern Ukraine proclaimed their regions independent Monday and called for a referendum on seceding from Ukraine — an ominous echo of the events that led to Russia's annexation of Crimea.

The Ukrainian government accused Russia of stirring up the unrest and tried to flush the assailants from some of the seized buildings, setting off fiery clashes in one city. Russia, which has tens of thousands of troops massed along the border, sternly warned Ukraine against using force.

In Washington, the U.S. said any move by Russia into eastern Ukraine would be a "very serious escalation" that could bring further sanctions. White House spokesman Jay Carney said there was strong evidence that some of the pro-Russian protesters were hired and were not local residents.

At the same time, the U.S. announced that Secretary of State John Kerry will meet with top diplomats from Russia, Ukraine and the European Union in a new push to ease tensions. The meeting, the first such four-way talks since the crisis erupted, will take place in the next 10 days, the State Department said.

Pro-Russian activists who seized the provincial administrative building in the city of Donetsk over the weekend announced the formation Monday of the independent Donetsk People's Republic.

They also called for a referendum on the secession of the Donetsk region, to be held no later than May 11, according to the Russian news agency Interfax.

A similar action was taken in another Russian-speaking city in the east, Kharkiv, where pro-Moscow activists proclaimed a "sovereign Kharkiv People's Republic," Interfax said.

It quoted the regional police as saying they later cleared the regional administration building, and the activists responded by throwing firebombs and rocks at the windows and setting tires ablaze. Local news reports said that the pro-Russian crowds then recaptured the building.

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Kharkiv Mayor Gennady Kernes said the activists also took control of the TV tower in Kharkiv and demanded it resume broadcasting banned Russian channels. But local police said the assailants later left.

Viktoria Syumar, a deputy head of Ukraine's Security and Defense Council, said Ukrainian special forces had driven pro-Russian assailants from Ukraine's Security Service headquarters in Donetsk.

Russia annexed Crimea last month, following a referendum called just two weeks after the Black Sea peninsula had been overtaken by Russian forces. Ukraine and the West have rejected the vote and the annexation as illegal.

The activists who occupied the government headquarters building in Donetsk blocked off the entrance with 6-foot barricades of car tires lined with razor wire.

Inside, dozens of people — almost all men, many of them wearing balaclavas and carrying clubs — stood around in groups. They refused to speak to journalists about their immediate plans.

As darkness fell, people in a crowd of a few hundred fired off a brief fireworks salute that was greeted by chants of "Russia, Russia!"

The Donetsk and Kharkiv regions — and a third Russian-speaking city besieged by pro-Moscow activists over the weekend, Luhansk — have a combined population of nearly 10 million out of Ukraine's 46 million, and account for the bulk of the country's industrial output.

Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of fomenting the unrest to create a pretext for sending troops in and taking another piece of Ukraine.

"The plan is to destabilize the situation. The plan is for foreign troops to cross the border and seize the country's territory, which we will not allow," Yatsenyuk said, adding that those taking part in the unrest had distinct Russian accents.

Swedish Foreign Minister Carl Bildt also said the events in eastern Ukraine were part of Moscow's "destabilization strategy."

"Those who thought that it ended with Crimea were wrong," Bildt said.

The Russian Foreign Ministry rejected the Ukrainians' allegations but reaffirmed its long-held demand that Ukraine change its constitution to turn the country into a federation with broader powers for provinces.

"If the political forces that call themselves the Ukrainian government continue to take an irresponsible attitude to the fate of the country and its people, Ukraine will inevitably face new difficulties and crises," the ministry said in a statement.

Russian Foreign Minister Sergey Lavrov warned the Ukrainian government against using force in response to the "legitimate demands" of people in eastern Ukraine.

Eastern Ukraine has a large population of ethnic Russians and was the base of support for ousted President Viktor Yanukovych, who fled to Russia in February after months of protests. Economic and cultural ties to Russia are strong here, and many are wary of the new government, which favors closer ties to the European Union.

In a video posted on the Internet, an unidentified pro-Russian activist in the Donetsk government headquarters asked Russian President Vladimir Putin to send peacekeeping troops into the region to support the protesters.

But a senior Russian lawmaker suggested Monday that such a move was not imminent.

Viktor Oserov, head of the defense committee in the Russian Parliament's upper house, said Moscow cannot send peacekeepers in without a U.N. Security Council resolution, according to Interfax.

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Jim Heintz and Vladimir Isachenkov in Moscow, Karl Ritter in Stockholm and Maria Danilova in Kiev contributed to this report.