Armed pro-Russian separatists have raised the Russian flag in the troubled eastern Ukrainian city of Slovyansk, deepening a stand-off with Moscow which, Kiev warned, was dragging Europe closer to a "gas war" that could disrupt supplies across the continent.

At least 20 men armed with pistols and rifles took over the police and security services headquarters in the city, about 150km from the border with Russia.

Officials said the men had seized hundreds of pistols from arsenals in the buildings. The gunmen replaced the Ukrainian flag on one of the buildings with the red, white and blue Russian flag.

Speaking shortly after the separatists seized the police station, Interior Minister Arsen Avakov said Ukrainian special forces had been dispatched to the scene.

"Our response will be very severe,"  Avakov wrote on his Facebook page.

"There is zero tolerance for armed terrorists," he added.

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Shortly after, the Ukrainian interior ministry also said the separatists had taken another security building in the restive city.

"The same group of armed men who seized the district police station also seized the Slavyansk Ukrainian Security Service (SBU) building," the regional interior ministry said in a statement.

In the industrial city of Donetsk, an AFP reporter said about 200 pro-Russian separatists armed with clubs and sticks stormed the city's police headquarters.

The protesters met no resistance, and a bus filled with a few dozen anti-riot police who quickly arrived at the scene were seen sporting orange and black ribbons symbolising support for Russian rule.

Witnesses said the men occupying the police headquarters were wearing the uniforms of Berkut, the uniform of Ukraine's feared but now-defunct riot police.

Bowing to demands from the pro-Russian separatists Donetsk police chief stepped down on Saturday.

"In accordance with your demands I am stepping down," police chief Kostyantyn Pozhydayev told protesters.

The latest takeovers by the gunmen come a day after Ukraine's prime minister, Arseniy Yatsenyuk, told leaders in Donetsk that he was willing to cede more power to the troubled eastern regions.

Eastern Ukraine has a high proportion of Russian-speakers and many of them fear that the acting government which took over when Viktor Yanukovich fled to Russia in February will repress them.

The Ukrainian government has accused Russia of fomenting unrest in eastern Ukraine in a bid to derail next months' presidential election in the country.

Source: Agencies