Government forces 'move on Sloviansk'
Ukraine is bringing back military conscription with immediate effect to deal with a spreading pro-Moscow insurgency in its east.
Interim president, Oleksandr Turchynov, has issued a decree to bolster Ukraine's defence capabilities.
He says Western-backed authorities in Kiev are "powerless" to stop pro-Russian separatists in the east from taking over public buildings.
Ukrainian government forces are reported to have launched an operation in the city of Sloviansk.
The city is a stronghold for pro-Russian separatists who are exerting increasing control in eastern Ukraine.
News agencies report gunfire, explosions and a military helicopter firing on the outskirts of the city.
But the BBC's Sarah Rainsford has spoken to separatists at checkpoints near the city who say there is no fighting in their sectors.
Russian television channels are saying that the city is being "stormed".
Earlier, Ukraine's acting President Olexander Turchynov reinstated military conscription to deal with deteriorating security in the east of the country.
Ukraine's interim prime minister said on Thursday his country was entering its "most dangerous 10 days" since independence in 1991 and was struggling to counter pro-Russian separatists on the verge of taking over the industrialized eastern heartland.
Arseniy Yatseniuk, in an interview with the Financial Times, accused Moscow of plotting to foment more clashes during the May Day holidays when nostalgia for Soviet victories and achievements tends to peak.
Pro-Russians strengthened their grip on the east of Ukraine on Thursday, storming the regional prosecutor's office in the town of Donetsk driving the police out and ransacking the building. The Kiev authorities fear the secessionists will put on a bigger show of strength on May 9, the commemoration of the Soviet Union's victory over Nazi Germany.
The move, announced in a decree, came as pro-Russia militants seized the regional prosecutor's office in the city of Donetsk, an industrial hub where a number of government offices have been seized in recent weeks.
Eastern Ukraine has a large Russian-speaking population and was a stronghold for President Viktor Yanukovych before he was overthrown by pro-Western protesters in February.
The crisis has plunged East-West relations to their lowest point since the Cold War.
On Thursday, German Chancellor Angela Merkel asked Russia in a phone call to President Vladimir Putin to help free foreign monitors held in eastern Ukraine.
The military observers were seized by pro-Russia separatists at a checkpoint in the flashpoint town of Sloviansk last Friday.
For his part, Mr Putin reiterated his call for Kiev to withdraw troops from the south-east to open the way for a national dialogue.
Mrs Merkel is due to meet US President Barack Obama in Washington on Friday to discuss the crisis in Ukraine
