The Ukrainian government says its special forces have engaged armed pro-Russian militia-men in a gunfight for control of key police buildings in the eastern city of Slovyansk.
The interior minister, Arsen Avakov, said at least one Ukrainian officer had been killed and five wounded as both sides suffered casualties in the exchange.
Earlier, Mr Avakov said his forces would be launching what he described as an “anti-terror” operation against the gunmen, issuing a warning for members of the public to keep clear.
The display of Ukrainian defiance comes despite Russia warning that it will not take part in a summit in Kiev next week if force is used against protesters - armed or not.
But Kiev has accused Russia itself of being involved in a bit to deepen violence and chaos across Ukraine. The government said Moscow is trying to undermine the legitimacy of presidential elections scheduled for 25 May.
The Russian news agency RIA reported that one protester was killed in the clashes. “On our side, another two were injured,” RIA quoted pro-Russian militant Nikolai Solntsev as saying.
Yesterday a pro-Russian militia took control of the security service offices and police headquarters in the city, calling for a break away from the control of the central government in Kiev.
Mr Avakov described the protests as “Russian aggression” and said that the “separatists” had opened fire on his troops as they assembled outside the security offices.
“An anti-terrorist operation has begun in Slovyansk. It is being directed by the anti-terrorist centre of the state security service. Forces from all the security units of the country have been brought in,” Interfax news agency quoted the minister as saying on his Facebook page.
“Pass it on to all civilians,” Mr Avakov's message read. “They should leave the centre of town, not come out of their apartments, and not go near the windows.”
Activists carrying automatic weapons seized government buildings in the city about 150 km (90 miles) from the Russian border on Saturday, and set up barricades on the outskirts of town.
Slovanysk’s mayor said the men who captured the police station were demanding a referendum on autonomy and possible annexation by Russia.
Reporters in the city said about a dozen women, who had been in the building when it was seized, were allowed to leave as soon as Mr Avakov's announcement was made known.
In addition to the actions of the pro-Russian militia, a group of around 100 civilians had set up a peaceful protest in front of the building – in support of those inside.
Meanwhile John Kerry, the US Secretary of State, spoke to Russia’s foreign minister Sergey Lavrov to condemn the developments in Slovansk, which he described as “orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea”.
The fall of key buildings in the city came a week after protesters in the nearby industrial hub, Donetsk, took over the regional government and declared a new “republic”. Yesterday also saw Donetsk’s police headquarters fall to activists.
And the pattern of capture and re-capture of government buildings has continued across eastern Ukraine in the past two weeks, including the major cities of Kharkiv and Luhansk.
The Russian foreign ministry has repeatedly dismissed any attempts by the West to blame it for what is going on. Mr Lavrov today said the crisis stemmed from the failure of the Ukrainian government to “take into account the legitimate needs and interests of the Russian and Russian-speaking population”.
He added that Russia may pull out of diplomatic talks due to be held in Kiev next week if Ukraine persists in using force against “residents of the southeast who were driven to despair”.
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