Fighting broke out early Friday around an eastern city in Ukraine that has become the focus of a pro-Russian insurgency as government troops attempted 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.
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.
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, an Associated Press reporter saw six Ukrainian armored vehicles parked on the side of the road.
An AP cameraman saw black plumes of smoke on the edge of the city. An emergency siren had sounded at dawn.
The center of Slovyansk appeared quiet but empty and tense Friday morning.
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 against committing 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.
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.