Pro-Russian separatists in the eastern Ukrainian city of Slovyansk say that Ukrainian security forces are trying to retake control of the city in a "large-scale operation."

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, BBC reports.

Vyacheslav Ponomaryov, the self-styled separatist mayor of Slovyansk, is saying that two Ukrainian helicopters have been shot down; one pilot had been killed and another captured.

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.

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