UPDATE (9:13 am EDT): Pro-Russian separatists have shot down a second Ukrainian military helicopter, the apparent result of shoulder-fired missiles, according to Ukraine's defense ministry, Interfax reported Friday.

Pro-Russian fighters battling for the eastern Ukrainian city of Slavyansk have shot down a helicopter and killed a pilot, as government forces launched an onslaught to retake control of the rebel-held city, Ukrainian officials said.

The Ukrainian military has regained control of a rebel checkpoint in the south of the town, Agence France-Presse said Friday, citing pro-Russian forces.

Kiev forces launched the counter offensive, which the pro-Russian fighters have termed a "full-scale onslaught," defying Russian President Vladimir Putin's warning that Ukraine should withdraw its military from the eastern and southern regions of the country.

Artillery fire and explosions were reported on the outskirts of Slavyansk early Friday, though the city of 160,000 was still relatively calm. Armored vehicles were seen taking positions and helicopters were circling.

Reuters said Ukraine's interior minister confirmed a helicopter pilot was killed and others were wounded by pro-Russians in Slavyansk.

The Ukrainian offensive is the first major assault on the eastern cities occupied by pro-Russian forces in the aftermath of the Crimea crisis and follows shortly after Kiev's decision to launch mandatory conscription to beef up its military.