Pro-Moscow activists barricaded inside government buildings in eastern Ukraine proclaimed their regions independent 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. Russia, which has tens of thousands of troops massed along the border, sternly warned Ukraine against using force.

In Washington, the US 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 US 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 of the independent Donetsk People’s Republic yesterday.

They also called for a referendum on the secession of the Donetsk region, which borders Russia, 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 declared themselves “alternative” regional legislators and 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.

Kharkiv Mayor Gennady Kernes said the activists took control of the broadcasting tower in Kharkiv and demanded it resume broadcasting Russian television channels that the Ukrainian government banned.

Viktoria Syumar, a deputy head of Ukraine’s Security and Defense Council, said Ukrainian special forces had driven the pro—Russian assailants from Ukraine’s Security Service headquarters in Donetsk, which they had seized earlier in the day.

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