A Ukrainian officer was killed with pro Russian armed men in the eastern city of Slavyansk in a shootout, the Interior Minister says.
Both sides suffered a number of casualties, said arsenic Abraham.
Slavyansk took over pro Russian troops on Saturday and were the target of at least four other cities, the prompt of Kiev, to start an "anti-terrorist operation".
The trouble throwing Moscow of the instigation of Kiev and the Western powers. The Kremlin denies the accusation.
Ukrainian forces military helicopter used in their offer to relocate Slavyansk demonstrators 
demonstrators burned tyres and their defenses strengthened, before the Government began operationUS officials said on Saturday there was a "concerted campaign" by forces with Russian support, to undermine the authorities in Kiev.
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David Stern BBC News, DonetskUkrainian authorities had said that she would militarily respond Russian invasion in the East of the country to all. Now, they are accused Moscow in an "Act of aggression" and orchestrated, if not actually perform the seizure of government buildings.
The question arises: this is a medium-sized war? Can somehow manage to avoid, or "Anti-terrorist operations" continue to the Government that this growing insurgency explosiveness to a total conflict?
Large parts of Eastern Ukraine are except slip control of Kiev. More and more police stations and Government buildings fall, unidentified armed to carry the Russian arms and look very similar to the Kremlin forces, Crimea took. The Government of Ukraine seems that there no choice whether violence have applied. The election, it seems is made for you.
Secretary of State John Kerry warned of "additional consequences" if Russia "De-escalation", not try and pull its troops back from the Ukrainian border.But Russia's Foreign Minister Sergei Lavrov said his inability to the Kiev Government, was responsible for the destiny of the country "demonstrated".
He had previously warned that any use of force in the Eastern Ukraine talks later this week due could fail crisis.
Colleagues from Russia were Ukrainian officials to meet the United States and the EU in Geneva on Thursday.
"Shoot to kill"Armed men took over the police stations and Government buildings in Slavyansk, Kramatorsk, Druzhkovka on Saturday.
Unconfirmed reports suggested, had assumed public buildings in two other cities - Mariupol and Yenakievo.
Similar accounts Slavyansk and Kramatorsk sprang on Saturday of armed men, dressed in camouflage in buses arrive and storm the police stations.
The security services have pro-Russian activists blocked Office in Luhansk 
in the city, Donetsk, have been occupied several public buildingsPro-Russian protesters continue their occupation of the main administration building in the capital of the region of Donetsk, who have held them for a week.
A protest leader told the BBC that the activists in Slavyansk action taken to support the sit-in of Donetsk.
BBC reporter in the Slavyansk said the armed were well organized and quickly established control throughout the city.
Checkpoints set up on major roads in the city.
There was heavy shots, when armed men in the police station in KramatorskMr Avakov characterized the actions of a "display of aggression by Russia".
Read the main story November 2013: President Viktor Yanukovych rejects an EU agreementDec: pro EU protests break out20 and 21 February 2014: clashes killed dozens in Kiev22 Feb: Mr Yanukovych flees; 27-28 Feb: Pro Russian armed take advantage of important buildings in Crimea.16 Mar:Voters vote for Crimea secede to controversial referendum: Russia later absorbed regionAPR: per Russia activists take over government buildings and police stations in the Eastern UkraineAnnouncing remain the operation to disable the activists, he warned people in their homes in Slavyansk."The separatists shoot to kill, without warning of the approaching special forces," he said,
Later he said Ukrainian forces was at a checkpoint on the way to the Slavyansk, was attacked and at least a policeman were killed and five others injured.
An unknown number of militants were also wounded, he said.
Witnesses to the police say there is no sign of clashes and the Centre of the city is quiet.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the fall of the Ukrainian pro-Russian President Viktor Yanukovych in February.
Are you in the Eastern Ukraine? Have you concerned by the unrest? You can experience by e-Mail haveyoursay@bbc.co.uk with the subject line "Ukraine".
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