The Ukrainian army has begun a special operation against pro-autonomy activists in the eastern town of Slavyansk.  

Ukraine's Defense Ministry has confirmed that two of its helicopters were shot down and the pilots killed in a major offensive to retake the eastern city of Slovyansk, RFE/RL reports.

Interior Minister Arsen Avakov said on Facebook the goals of the offensive are to "free the hostages, lay down their arms, and free administrative buildings, and restore the normal functioning of the town's administration."

Slovyansk, which has a population of some 140,000 people, has been under the control of the pro-Russian separatist forces since last month.

Ukrainian troops had taken control of nine checkpoints on roads around the city.

A plume of black smoke was seen rising on the outskirts of the city. Correspondents report gunfire and explosions. An AFP reporter saw a column of eight Ukrainian armored vehicles breaching a rebel-held checkpoint just south of Slovyansk.

The rebels have detained Slovyansk's mayor, several journalists, three Ukrainian Security Service members, and are also holding a team of OSCE military observers.