WATCH: Pro-Russian protesters storm building in Ukraine

ABOVE: Pro-Russian protesters, brandishing iron bars and bricks, attacked two buildings in Ukraine’s Donetsk on Saturday, in an apparent retaliation for fatal clashes in Odesa. Mobs of masked men targeted a building owned by Donetsk’s appointed governor Serhiy Taruta, first throwing bricks at the front entrance before storming the building.

MOSCOW – European military observers who were held more than a week by insurgents in eastern Ukraine walked free Saturday, with Kyiv insisting the release proves Russia is fomenting unrest in Ukraine – as Moscow touted the insurgents as courageous humanists.

Despite the release, tensions in Ukraine heightened sharply after at least 42 people died in clashes between government supporters and opponents in the Black Sea port of Odesa on Friday. On Saturday, news reports claimed fighting broke out in the city of Kramatorsk, about 15 kilometres south of Slovyansk.

The Odesa clash began with street fighting between two sides in which at least three people were reported killed by gunfire, then turned into a grisly conflagration when government opponents took refuge in a building that caught fire after protesters threw firebombs inside.

At least 36 people died in the fire, according to the emergencies ministry. An Interior Ministry statement gave the overall death toll for the day at 42, but did not give a breakdown.

The city’s police chief, Petr Lutsyuk, on Saturday issued a statement calling for calm in the city of about 1 million, but hours later he was fired by Interior Minister Arsen Avakov.

Peskov, the spokesman for Russian president Vladimir Putin, said the bloodshed demonstrated the acting government’s tolerance of or collusion with nationalist extremists and had driven efforts to resolve the crisis into a dead end.

The Ukrainian Foreign Ministry blamed pro-Russia adherents for setting off the clash, didn’t mention the fire, and claimed the “events in Odesa show that separatists’ subversive activities in Ukraine are doomed to failure.”

Odesa, some 550 kilometres southwest of Slovyansk, had not previously seen significant confrontations in Ukraine’s crisis, and the deaths there suggested that violent unrest could spread far from the relatively compact area in the east where it has been concentrated so far.

Odesa is the major city between the Crimean peninsula, which Russia annexed in March, and the Moldovan separatist region of Trans-Dniester where Russia has a military peacekeeping contingent. Some analysts speculate that Russia ultimately aims to take control of a huge swath of Ukraine from Trans-Dniester to the east.

Leonard reported from Slovyansk, Ukraine. Kirsten Grieshaber in Berlin contributed.