DONETSK, Ukraine — Tensions escalated in eastern Ukraine on Wednesday, with pro-Russian gunmen storming City Hall in the sprawling city of Donetsk and a cluster of Ukrainian troops meant to be restoring order in the region apparently defecting to the side of separatists.

The events suggested the challenge ahead for the pro-Western Ukrainian government on the second day of a campaign to quell the restive east, and came as Russian President Vladimir Putin warned that Ukraine was on the verge "of a civil war."

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A line of combat vehicles flying Russian flags rolled Wednesday morning into Slovyansk, a city of 120,000 where separatists have set up roadblocks since Saturday. One soldier named Andrei, speaking to the Associated Press, identified the men as part of the 25th Brigade of Ukraine's airborne forces that had switched to the side of pro-Russian forces. The troops in green camouflage and packing automatic weapons and grenade launchers received a warm welcome from local separatists, AP said. The report could not be immediately verified.

Around 10 a.m. local time, a squad of separatists backed by seven masked gunmen in camouflage stormed the seat of Donetsk's mayor and local council.

By Wednesday afternoon, more than 40 men, some masked and heavily armed, were occupying the building but still allowing workers and local officials to go about their business inside City Hall.

The scene was calmer than earlier in the morning, when suited bureaucrats were running back and forth to vehicles in an attempt to save files and computers. City workers were shuffling to and from meetings under the watchful gaze of camouflage-clad militants who loitered in the corridors, many clutching automatic weapons. Offering glaring evidence of the Kiev government's tenuous grip on the region, a few local police officers casually strolled outside without attempting to intervene.

The pro-Russian militants who took over City Hall said they were separate from a similar group that occupied the regional headquarters in this city of nearly 1 million 10 days ago, but they issued at least one similar demand. They called for a referendum on May 11 with two questions: whether the populace agreed with the creation of a new "Donetsk People's Republic" and, if so, whether it should be part of Ukraine or Russia.

"Why should we consider Russia a hostile state?" asked Alexander Zakharchenko, a militant commander at City Hall. "They are the closest people to us in the world." He commands the Donetsk branch of a group called Oplot, a pro-Russian movement that started as a fight club of young men in the Ukrainian city of Kharkiv, to the north.

Russia said it was still planning to attend four-party talks aimed at resolving the Ukrainian crisis in Geneva on Thursday and would use the meeting to press for the launch of constitutional reform in Ukraine.

READ MORE: Ukrainian troop defections escalate tensions in eastern Ukraine