Ammon News - AP - Ukrainian special forces have been sent to an eastern city where armed pro-Russia men seized a police headquarters and the Security Service office a day earlier, the interior minister said Sunday.

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.

Arsen 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." Avakov called on local residents to remain calm and stay at home.

An Associated Press reporter saw no signs of any shots fired at the police station which was surrounded by a reinforced line of barricades. Unlike on Saturday, the men patrolling the barricades were largely unarmed. One of the guards who asked not to be identified denied reports of fighting at the police station.

Armed camouflaged men were guarding a checkpoint at the main entrance into the city, not allowing anyone to enter. The claims of gunfire inside Slovyansk couldn't be immediately verified, and.

In a phone call with Russian Foreign Minister Sergey Lavrov, U.S. 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.

The Russian Foreign Ministry debunked Kerry's claims while Lavrov blamed the crisis in Ukraine on the failure of the Ukrainian government "to take into account the legitimate needs and interests of the Russian and Russian-speaking population," the ministry said. Lavrov also warned that Russia may pull out of next week's Ukraine summit if Kiev uses force against "residents of the southeast who were driven to despair."

Swedish Foreign Minister Carl Bildt, who is in Ukraine this weekend, condemned the unrest in a Twitter post as "a coordinated armed action to seize control over key parts of Eastern Ukraine" which "would not have happened without Russia."

In Slovyansk, the mayor said Saturday the men who seized the police station were demanding a referendum on autonomy and possible annexation by Russia. Protesters in other eastern cities have made similar demands after a referendum in Crimea last month in which voters opted to split off from Ukraine, leading to annexation by Russia.

The interior minister overnight reported an attack on a police in the nearby city of Kramatorsk. A video from local news web-site Kramatorsk.info showed a group of camouflaged men armed with automatic weapons storming the building. The news web-site also reported that supporters of the separatist Donetsk People's Republic have occupied the administration building, built a barricade with tires around it and put a Russian flag nearby.

Regional news website OstroV said three key administrative buildings have been seized in another city in the area, Enakiyeve.

In the regional capital Donetsk on Saturday, witnesses said the men who entered the police building were wearing the uniforms of the Berkut, the feared riot police squad that was disbanded in February after Yanukovych's ouster. Berkut officers' violent dispersal of a demonstration in Kiev in November set off the mass protests that culminated in bloodshed in February when more than 100 people died in sniper fire. The acting government says the snipers were police.

It wasn't immediately clear if the men who occupied the Donetsk police building had made any demands, but the Donetsk police chief said on national television that he was forced to offer his resignation.