Ukraine troops kill Five Pro-Russian Separatists in Sloviansk
A member of the Ukrainian special forces takes position at an abandoned roadblock in the eastern Ukrainian city of Sloviansk AFP

Ukraine has said pro-Russian fighters battling for the eastern 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.

The Ukrainian military has regained control of a rebel checkpoint in the south of the town according to Agence France-Presse, 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 on Friday, though the city of 160,000 people was still relatively calm, reports said. Armoured vehicles were seen taking positions and helicopters were circling.

Pro-Russian forces shot down two helicopters, killing one pilot and capturing another, AFP reported, quoting Vyacheslav Ponomarev, who has been appointed the mayor of Slovyansk by the rebels.

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.