Pro-Russian separatists shot down two helicopters in a key eastern Ukrainian city, and fighting in the port city of Odessa triggered a fire that killed dozens, as the embattled nation moved closer to the brink of civil war.

Interim Ukraine President Oleksandr Turchynov said "many" pro-Russia rebels have been killed, injured and arrested in a major offensive to regain control of Slavyansk, though it was not clear if the Kiev-backed forces had succeeded. Russia reacted angrily to the offensive by Ukrainian security forces, calling for an emergency meeting of the UN Security Council after a spokesman for President Vladimir Putin warned it "effectively destroyed the last hope for the implementation of the Geneva agreements."

In the Black Sea port city of Odessa, Ukraine's third-largest city, a fire that broke out in a trade union building amid clashes killed 31. Fighting there represented another ominous milestone in the conflict that threatens to become a full-blown civil war, as Odessa holds huge historic significance for Russians and Ukrainians alike.

A Ukrainian Defense Ministry spokesman told Fox News that two Ukrainian helicopters were shot down and two of their crewmembers were killed and several Ukrainian soldiers were injured in the fighting around Slovyansk.

Residents of the city of 130,000 in the divided province of Donetsk were warned to stay indoors as a Ukrainian "anti-terrorist operation" was mounted. Two Mi24 helicopters were taken down with mobile surface-to-air missiles, killing two military officers and injuring others, according to the Ukrainian Defense Ministry website. Another army helicopter, an Mi8, was damaged but no one was hurt, it said.

The Ukrainian Security Service said its forces were fighting "highly skilled foreign military men" in Slovyansk. The clash seemed to counter Russia's claims that the city is under control of civilians who took up arms.

Stella Khorosheva, a spokeswoman for the pro-Russian militants, said one of their men was killed and another injured. She offered no further details.

A Reuters photographer said he saw a military helicopter open fire on the outskirts of the town and a reporter heard gunfire. Pro-Russia forces told Reuters they were under attack and that at least one helicopter had been shot down.

Vyacheslav Ponomarev, the insurgency-appointed mayor of Slovyansk, told Fox News that three Ukrainian helicopters had been shot down. He said that one pilot was killed and another had ran away from the scene.

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.

By nightfall, Ukrainian troops and armored personnel carriers blocked all major roads into Slovyansk, and the central part of the city remained in the hands of pro-Russia gunmen, according to Associated Press journalists inside. Most shops were closed, and the few that were open were crowded with customers stocking up on supplies.

It 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.

The armed element of the insurgency focused on Slovyansk, a city 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.

Dmitry Peskov, Putin's spokesman said the Kremlin had sent an envoy to Ukraine's southeast to negotiate the release of foreign military observers who were captured by pro-Russian militia in Slovyansk.

In comments to Russian news agencies, Peskov did not specify where Vladimir Lukin was sent to but said the Kremlin has not been able to get in touch with him since Ukraine launched the offensive.

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 on Thursday, Putin said the removal of military units was the "main thing," but it was unclear if that could be construed as an outright demand.

“Putin emphasized that it was imperative today to withdraw all military units from the southeastern regions, stop the violence and immediately launch a broad national dialogue as part of the constitutional reform process involving all regions and political forces,” the Russian government said in a statement, according to The New York Times.

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.

The Associated Press contributed to this report.