Slovyansk, Ukraine: Two children have reportedly been killed here in Slovyansk where automatic gunfire can be heard in the town centre.

Armed men manning a checkpoint at the town's entrance have ordered Gulf News away. Well-armed men in military uniforms took over the police building Saturday.

Witnesses say the two children were shot as a Ukrainian security force helicopter was fired upon.

Ukrainian special forces were deployed here Sunday, the interior minister said.

The unrest in Slovyansk and the nearby major industrial city Donetsk were the latest shows of spiralling 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."

The town is sealed off and Gulf News can't get in verify the reported deaths.

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

In a phone call with Russian Foreign Minister Sergey Lavrov, US 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."

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.