Residents in the eastern city of Slovyansk were warned to stay indoors Sunday as Ukraine’s interior minister announced forces would attempt to regain control of official buildings from pro-Russian militant separatists.
Military helicopters were seen over the besieged buildings, but it was not clear if any clashes had taken place on the ground.
Pro-Russian groups on the ground have been preparing heavily-armed barricades in the city, and any attempt by Ukrainian forces to regain control is likely to be met with violence.
"Pass it on to all civilians: they should leave the center of town, not come out of their apartments, and not go near the windows," Interfax Ukraine news agency quoted Arsen Avakov as saying.
The unrest in Slovyansk and the nearby major industrial city Donetsk were the latest shows of spiraling anger in eastern Ukraine, which has a large Russian-speaking population and was also the support base for Viktor Yanukovych, the Ukrainian president ousted in February following months of protests in Kiev, the capital.
Ethnic Russians in Ukraine's east widely fear that the new pro-Western Ukrainian government will suppress them.
In a phone call with Russian Foreign Minister Sergey Lavrov, Secretary of State John Kerry "expressed strong concern" that the attacks "were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea," according the State Department.
Kerry "made clear that if Russia didn't take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences," the department said.
Avakov wrote on his Facebook page that the men who seized the buildings in Slovyansk had opened fire on the approaching troops and described the unrest as "Russian aggression" – however the reports of gunfire could not be verified.