A group of pro-Russian men armed with automatic weapons have taken control of the police headquarters in the Ukrainian city of Kramatorsk, 150 km from the Russian border, local officials and witnesses said.
An organised military unit of over 20 men wearing matching military fatigues and carrying automatic weapon took over the building around 1700 GMT after arriving on at least two buses.
Our response will be very severe. There is zero tolerance for armed terrorists.
Video footage showed the men taking orders from a commander and shooting from automatic rifles as they approached the building.
Ealier a gunbattle erupted after security forces engaged the men who attacked the station, the interior minister said.
"Unknown men opened fire at the district police station. The police are firing back. They have been exchanging fire," Arsen Avakov said late on Saturday on his Facebook page.
The seizure came hours after armed pro-Russian separatists raised the Russian flag in the troubled eastern 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 Slovyansk, 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, 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.
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.
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Meanwhile, the Donetsk police chief stepped down on Saturday, bowing to demand from the pro-Russian separatists.
"In accordance with your demands I am stepping down," police chief Kostyantyn Pozhydayev told the 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.
In a show support for the embattled prime minister, US Vice President Joe Biden will travel to Ukraine on April 22 to stress US support for Kiev and help improve its energy security.
While in Ukraine, Biden will work "to assist Ukraine in moving forward on constitutional reform, decentralization, anti-corruption efforts and free and fair presidential elections on May 25th," the White House said.
Washington has repeatedly urged Moscow to de-escalate tensions and withdraw Russian troops from Ukraine's eastern border in the worst East-West standoff since the Cold War.