Shooting reportedly started at a checkpoint, which was established by protesters Saturday evening on the outskirts of the city.

In the skirmishes, one of the troops from Kiev was killed and five others were injured, coup-appointed Interior Minister Arsen Avakov reported on his Facebook page, which he regularly uses to report on his ministry’s activities.

The officer killed was a member of the Ukrainian Security Service, he said, adding that among those injured is the commander of the SBU’s Anti-Terrorist Center.

The interior ministry in Kiev said both sides had suffered casualties during a raid in Slaviansk.

"There are dead and wounded on both sides," Interior Minister Arsen Avakov wrote on his Facebook page. "On our side -- a Ukrainian Security Service officer... On side of the separatists -- an unidentified number... The separatists have started to protect themselves using human shields."

Any operation to dislodge the armed separatists risks tipping the stand-off into a new, dangerous phase because Moscow has warned it will act to protect eastern Ukraine's Russian-speakers if they come under attack.

Avakov said security units from across the country had been brought in to launch an "anti-terrorist operation" to re-assert Kiev's control in Slaviansk.

A Reuters’ reporter in the city, about 150 km (90 miles) from the Ukraine-Russia border, said two military helicopters were flying over the town's police headquarters where separatists were holed up.

"Pass this on to all civilians: they should leave the center of town, not come out of their apartments, and not go near the windows," Avakov wrote on his Facebook page.

In the nearby town of Kramatorsk, militants exchanged gunfire with police late on Saturday, though there was no confirmation any one had been hurt.

Novosti Donbass, a local Internet news site, said that government forces had taken down two rebel barricades at entry points into Slaviansk, but there was no independent confirmation of this.

Residents in the town did not appear to have heeded the appeal by Avakov and families were out on the streets on their normal Sunday business.

Representatives of Russia, Ukraine, the United States and the European Union are due to meet in Geneva on Thursday for the talks.

Russia is expected to argue for a revamp of Ukraine's constitution to give a large degree of autonomy to eastern Ukraine, something Kiev and its Western backers reject.

NTJ/HH