Ukrainian special forces exchanged gunfire with a pro-Russia militia in an eastern city today, killing one security officer and wounding five others, the interior minister said.
It was the first reported gunbattle in east Ukraine, where armed pro-Russia men have seized a number of law enforcement buildings in recent days.
Arsen Avakov said in a Facebook post that a Security Service officer was killed in Slovyansk, where the police station and local Security Service office were seized a day earlier by camouflaged armed men. He also reported an unclear number of casualties among the militia.
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.
Mr Avakov has described the unrest as "Russian aggression."
In an earlier post, he said the men who seized the buildings in Slovyansk had opened fire on Ukrainian special forces sent to the city today. He called on 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 synchronised, similar to previous attacks in eastern Ukraine and Crimea," according to the State Department.
Mr 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 rejected the claims, while Mr 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.
Mr 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 yesterday 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.