SLOVYANSK, Ukraine (AP) — Two helicopters were shot down and two Ukrainian soldiers killed near the eastern Ukrainian city of Slovyansk Friday, as separatists battled government forces who apparently launched a major offensive on the city that has become the focus of an armed pro-Russian insurgency.

Gunfire and blasts were heard early Friday around the city, as separatist leaders claimed that government troops had assayed a military assault in an attempt to retake control.

The fighting, confirmed by both sides, appeared to be the first major assault against the insurgents who have seized police stations and other government buildings in about a dozen cities in southeastern Ukraine.

Two Ukrainian helicopters were shot down and their pilots were killed, Ukraine’s Defense Ministry said.

The Ukrainian Security Service said its forces were fighting “highly skilled foreign military men” in Slovyansk.

The Security Service said one helicopter was shot down with a surface-to-air missile, which they said countered Russia’s claims that the city is under control of civilians who took up arms.

A video posted to YouTube purported to show one of the downed helicopters in flames.

The Ukrainian interior minister, Arsen Avakov, said on his official Facebook page that government troops met fierce resistance, but had managed to take control of nine checkpoints on roads around Slovyansk.

Vyacheslav Ponomarev, the insurgency-appointed mayor of Slovyansk, said self-defense forces had shot down two helicopters, killing one pilot and and capturing the other.

Ponomarev said no Ukrainian troops could be seen in the city.

An AFP journalist said Ukrainian soldiers had taken control of a rebel checkpoint just to the south of the town. Eight armored vehicles and several soldiers were seen taking position there and warned off anybody trying to approach.

Sporadic small-arms fire was heard at a distance. Two explosions were heard.

The official spokesman for the military wing of the pro-Russian forces, who would give only his first name, Vladislav, said fighting had broken out at several points around the city. He said government armored vehicles were seen on roads leading into Slovyansk and claimed that Ukrainian troops had made incursions into the city itself.

Details of some of these claims could not be independently confirmed.

An Associated Press cameraman saw black plumes of smoke on the edge of the city. An emergency siren had sounded at dawn.

A helicopter was earlier seen circling low, said the rebels, who had set a roadblock on fire to produce cloaking smoke.

“This is a full-scale attack,” said an insurgent spokeswoman in the town.

Slovyansk itself was relatively calm, but the town’s church was ringing its bells to alert the population of 160,000.

The rebel leadership of the town has been warning for several days that the Ukrainian military was preparing an assault.

If the Ukrainian military action is confirmed, it would be the first major assault against the insurgents, who have seized police stations and other government buildings in about a dozen cities in southeastern Ukraine.

The armed element of the insurgency is focused on Slovyansk, a city 160 kilometers (100 miles) west of Russia in which seven European military observers from the Organization for Security and Cooperation in Europe remain held by pro-Russia gunmen.

Russian President Vladimir Putin said Thursday that Ukraine should withdraw its military from the eastern and southern regions of the country.

Hours later, Ukraine’s acting president ordered that the military draft be renewed, citing “threats of encroachment on the nation’s territorial integrity” and interference by Russia in its internal affairs.

Moscow has consistently denounced Ukrainian security forces’ largely ineffectual operation against the eastern insurgents and warned they should not commit violence against civilians.

In a telephone conversation with German Chancellor Angela Merkel, Putin said the removal of military units was the “main thing,” but it was unclear if that could be construed as an outright demand.

Oleksandr Turchynov’s conscription order marked a turnaround for the country, which last year announced plans to end military conscription in favor of an all-volunteer force. His order did not specify where conscript-bolstered forces could be deployed. The renewal of military conscription affects only men 18 to 25 years old.

Earlier in the week, the acting president said police and security forces had been effectively “helpless” against insurgents in the Donetsk and Luhansk regions, the heart of the unrest, and that efforts should be focused on preventing the instability from spreading to other parts of the country.