SLOVYANSK, Ukraine — Gunfire and blasts were heard early Friday around an eastern city in Ukraine that has become the focus of an armed pro-Russian insurgency, whose leaders claimed that government troops had assayed a military assault in an attempt to retake control.

Vyacheslav Ponomarev, the insurgency-appointed mayor of Slovyansk, said self-defense forces had shot down two helicopters, killing one pilot and capturing the other. He said no Ukrainian troops could be seen in the city.

The official spokesman for the military wing of the pro-Russian forces, who will 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 these claims could not be independently confirmed.

On the road leading into Slovyansk from Kramatorsk to the south, trucks were parked sideways to block the road.

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

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.