Ukrainian troops launched a dawn raid to try to retake the rebel stronghold of Slaviansk today, surrounding the eastern town and sending in military helicopters, one of which was shot down by pro-Russian rebels, killing its pilot.

Accusing the pro-Moscow separatists of using mercenaries, Interior Minister Arsen Avakov said the offensive was answered with heavy artillery, with the rebels using grenade and portable anti-aircraft missile launchers to bring down the plane.

Kiev has accused Moscow of financing and arming the separatists, who have seized buildings across Ukraine's eastern Donbass coal and steel belt and demand a referendum on independence on May 11 for at least two eastern regions.

Russia denies the charge, describing the separatists as Russian-speakers defending their rights from a possible assault by Kiev's pro-Western leaders.

Pro-Russian separatists in Slaviansk, the eastern Ukrainian where the rebels are most firmly in control, described the operation as "large-scale", and said they had taken one pilot prisoner.

The fighting began at 4am (0100 GMT), officials and local residents said.

Ukrainian officials, who have been criticised for being slow to act to stop the rebels, launched an "anti-terrorist" operation in early April but have resisted large-scale measures so far.

The rebels have seized official buildings in major centres and small towns across the Donbass, often with little or no opposition from local police.

Kiev has acknowledged it has little control over some of the police forces in the east, threatening the status of a presidential election due on May 25 which some in Kiev hope will draw a line under what has become a messy transitional period since President Viktor Yanukovich was ousted in February.