The Ukrainian forces started an operation against the separatists in Slaviansk early Friday morning, Ukrainian and Russian media report. 

Ukrainian Air Force combat helicopters are in the sky above the east Ukrainian city of Slaviansk, the city is fully blocked, fighting is underway, a representative of the militia told ITAR-TASS by phone on Friday. 

He claims two helicopters were downed, with one pilot dead and another injured and taken to the city hospital for treatment. 

Arsen Avakov, Ukraine's interior minister, said on his Facebook page, that interior ministry anti-terrorist units, the National Guard and the armed forces have seized control of nine roadblocks and Slaviansk is fully surrounded. 

He confirmed the downing of one helicopter and the death of the pilot.  

On his page Avakov wrote that the Ukrainian forces were facing “professional mercenaries” who use heavy weapons, including grenade launchers and portable anti-aircraft units.

According to Avakov, the “terrorists” are shooting from residential buildings and urged the residents of Slaviansk and Kramatorsk to remain at  home, stay away from the windows and avoid travelling. 

“The operation continues as planned,” Avakov wrote.