KIEV, Ukraine -- Undeterred by mayhem and the biggest death toll in a single day since the February 20 slaughter of pro-European protesters in Kiev’s Independence Square, the Ukrainian government has resumed operations against pro-Russian separatists in their eastern Ukraine strongholds. The move risks heightening the calculated fury of Moscow and offering a pretext for invasion by Russian troops massed along the border.

Across this tortured country there is deepening anxiety, with Ukrainians feeling this May Day weekend has marked a defining moment, and the sense of foreboding is mixed with shock at the news that Ukrainians are now burning Ukrainians. At least 40 people died in Odessa on Friday, most of them pro-Russian separatists killed as they sought to flee a burning trade union building. Many choked to death or suffered fatal injuries leaping from upper floors.

The high loss of life in Odessa, a southern sea port most Ukrainians associate with balmy summer vacations, increases the chances that Russia’s Vladimir Putin will make good on his threat to invade Ukraine to protect ethnic Russians. Russian officials have warned of “catastrophic consequences” and they say the events of the past 24 hours have banged a final nail into the coffin of the Geneva accord, an agreement reached last month among diplomats from Russia, Ukraine, the United States and Europe aimed at de-escalating the Ukraine crisis.

The only good piece of news today in this spiraling crisis was the announcement that half a dozen international observers seized by pro-Russians in the town of Slovyansk in eastern Ukraine last week were freed, along with five Ukrainian officers grabbed alongside the military monitors from the Organization for Security and Co-operation in Europe. The separatists insisted to the end that the observers were “NATO spies.” The separatists had sought to use them as bargaining chips in a prisoner exchange with Kiev. A Russian envoy reportedly played a role persuading the rebels in Slovyansk to let them go. The OSCE confirmed the release via Twitter.

The observers’ liberation did not halt a resumption of Ukrainian military operations by units drawn from interior and defense ministry forces on the outskirts of Slovyansk, the focus of pro-Russian insurgency in the area. Anti-separatist operations also expanded on Saturday to the neighboring and larger town of Kramatorsk, where anti-terrorist units last week were virtual prisoners, coming under grenade attacks from the pro-Russian militants.

Ukraine’s Interior Minister Arsen Avakov vowed on his Facebook page today, “We will not stop.” He said the effort to reassert control over eastern Ukraine “continued at dawn,” with the quick capture of a television mast in Kramatorsk, 20 kilometers from Slovyansk.

But despite claims from the country’s acting president on Friday that half of Slovyansk was in government hands, Ukrainian units have not tried to advance into the center of the city of 130,000, and according to local residents and media reports they still are not in control of all separatist checkpoints around the outside of the city, especially to the south. On Friday, pro-Russian separatists downed two military Mi24 helicopters, killing two servicemen and injuring others, according to Ukrainian defense officials. A third helicopter was damaged.

At a meeting of the United Nations Security Council yesterday Russia demanded an end to Ukrainian military operations in eastern Ukraine, where separatists have seized more than 30 government buildings in more than a dozen towns. Western powers and Kiev accuse Moscow of funding and fomenting the insurgency, seeking to create the pretext for an eventual intervention.

That pretext may have come with the violence that erupted in the port city of Odessa on yesterday. The full details of the events engulfing the city still remain unclear but running battles between separatists and Kiev loyalists flared after a rally of pro-Ukrainian football fans was attacked by separatists, some of whom wielded handguns, which left at least four dead and ended in the torching of a trade union building as the opposing sides exchanged Molotov cocktails. Police say 160 were arrested but locals criticize them for standing by for most of the day while the battles and skirmishes raged on.

Videos posted online overnight show separatists sitting on ledges trying to escape the fire and thick smoke. Pro-Russia fighters can be seen mounting a defiant last stand from the burning building, throwing anything that came to hand onto the attackers below.

Dazed and injured pro-Russia activists who stumbled out of the building were bundled into police vans, some after being assaulted and jeered by the crowd. Local Ukrainian media interviewed several members of Right Sector, an unsavory ultranationalist organization that plays right into Moscow’s propaganda campaign about fascists and crypto-Nazis taking over in Kiev. They said they are determined to sweep Odessa clean of pro-Russian militants.

Shots were fired from the rooftop of the burning building and paramedics said at least five people were killed by gunfire.

Ukraine’s Deputy Foreign Minister Danylo Lubkivsky told the BBC that Russia was to be blamed for the violence because it infiltrated operatives and provocateurs into the country to direct the pro-Russian unrest. “The security situation is threatened by Russian special agents,” he said.

Russian television outlets focused their coverage on the violence in Odessa, interviewing survivors from the fire who accused members of the ultranationalist Right Sector group for the loss of life. One protester told the Kremlin-controlled Russia Today: “When we were finally able to jump out of the window from the second floor, we were met by the Right Sector radicals. They beat us using bats and chains.”

Russian Twitter accounts and the country’s blogosphere have seen a torrent of demands for Russian intervention.