Thousands of pro-Russian protesters assaulted Odessa's police headquarters Sunday, days after deadly clashes and a fire there killed dozens of their comrades in what Kiev charged was a Russian plot to "destroy Ukraine."

The unrest in the southern port city threatened a new front in the Ukrainian government's battle against pro-Moscow militants, with an expanded military operation under way in the east against gunmen holding more than a dozen towns.

Ukraine vowed on Sunday to broaden its operation against pro-Russian rebels as the crisis-hit country observed a second day of mourning after violence that left more than 50 people dead.

National Security and Defense Council chief Andriy Parubiy said the armed forces would expand the "active stage of the operation in other towns where extremists and terrorists are carrying out illegal activities".

Flowers, candles and photos of the dead pile up outside a charred building in the scenic Ukrainian port city of Odessa where anger simmers a day after brutal clashes claimed 42 lives.

Under a warm spring sun, several thousand gathered outside the blackened trade union building, where dozens perished in an inferno that marked the culmination of a day of confrontation.

The head of an OSCE team released by rebels in east Ukraine on Saturday expressed his "deep relief" after an ordeal that lasted more than a week.

"It is happiness, a deep relief," German Colonel Axel Schneider told a small group of journalists on the road outside of the Ukrainian city of Donetsk.

Russian President Vladimir Putin's spokesman said it would be absurd to conduct snap polls in Ukraine amid the raging violence, saying the Kremlin no longer had any influence over rebels in the country's east.

"We do not understand what elections in Kiev they are talking about in European capitals and Washington," Dmitry Peskov was quoted as saying by Russian news agencies.

Seven European OSCE inspectors were freed on Saturday in a flashpoint town in east Ukraine, where surrounded pro-Moscow rebels are battling a fierce military assault amid a soaring national death toll.

The unexpected release was a bolt of good news in Ukraine's startling descent into chaos, after a bloody day in which more than 50 people died -- most of them in a horrific inferno in the southern city of Odessa.

More than 30 people were killed in a "criminal" blaze in Ukraine's southern city of Odessa, as violence spread across the country during the bloodiest day since Kiev's Western-backed government took power.

Ukraine's interior ministry said at least 31 people had died in the fire Friday, with local media reporting that pro-Russian militants were believed to have been in the burning building at the time.

Switzerland announced Friday that it had added the names of 15 top Russian officials and pro-Moscow separatists to a blacklist of individuals seen as prime movers in the Ukraine crisis.

The move by Switzerland, which is not a member of the European Union, matched the sanctions imposed against the same individuals by Brussels on Monday.

Russia is seeking to test NATO and members of the transatlantic alliance must increase military spending in the face of Moscow's challenge, U.S. Defense Secretary Chuck Hagel warned Friday.

The 28 members of NATO have responded to Russia's intervention in Ukraine with "resolve," Hagel said, "but over the long term, we should expect Russia to test our alliance's purpose, stamina, and commitment."

The United States and Germany delivered a firm warning to Russia on Friday that it would face direct and painful economic sanctions if Ukrainian elections later this month are disrupted.

U.S. President Barack Obama and German Chancellor Angela Merkel delivered the statement after talks at the White House appeared to bring the prospect of "sectoral" economic sanctions closer than ever before.