ODESSA, Ukraine - Thousands of pro-Russian protesters attacked Odessa's police headquarters on 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.
However, in a potentially significant diplomatic move on Sunday evening, the Kremlin announced that the head of the Organisation for Security and Cooperation in Europe (OSCE), Didier Burkhalter, will visit Moscow on Wednesday for talks on the crisis.
The meeting was announced following a phone call between Russian President Vladimir Putin and German Chancellor Angela Merkel, who later said Russia and the OSCE would discuss setting up a national dialogue in Ukraine.
"Putin and Merkel stressed the importance of effective international action -- especially by the OSCE -- in reducing the tensions in Ukraine," Russia said in its statement.
Earlier, Ukraine's Prime Minister Arseniy Yatsenyuk accused Russia of executing a plan "to destroy Ukraine and its statehood".
He was in Odessa to observe mourning for the 42 people who died there in clashes and the fire on Friday -- most of them pro-Russian militants.
The unrest shaking the Black Sea city of one million people, he said, aimed "to repeat in Odessa what is happening in the east of the country".
In an effort to head off any retribution on the streets for Friday's bloodshed, Yatsenyuk sacked Odessa's police chiefs and ordered an inquiry.
The police in the headquarters managed to calm the crowd outside by releasing 67 pro-Russian militants they were holding, nearly half the 150 total who had been arrested on Friday. One person, though, was reported wounded by gunshot in the city.
Three loud explosions were also heard by AFP journalists after 8:00 pm on Sunday (1700 GMT), but a spokeswoman for the rebels, Stella Khorocheva said "the fighting was over".
Although Moscow has admitted sending troops into Crimea ahead of annexing the strategic peninsula in March, it denies having a hand in Ukraine's unrest in the east and in Odessa.
Instead it blames the Kiev government and its Western backers for the carnage.
Moscow has also demanded a halt to the Ukrainian military offensive in the east, saying it has received "thousands" of calls for help from the population there for it to intervene.
Tens of thousands of Russian troops have been parked on Ukraine's border for two months, ready for an invasion that Putin says he has a right to launch -- but "hopes" he won't have to.
But Ukrainian officials have pushed on with the operation, determined to crush the pro-Kremlin rebels.
Late Sunday, a spokeswoman for rebels in the insurgent-controlled bastion of Slavyansk said "the town is completely surrounded".
AFP confirmed observing seven armoured vehicles blocking the last main route out, which is the road to the regional hub of Donetsk.
Ukrainian authorities have already put all armed forces on "combat alert" and brought back conscription as the risk of invasion looms.
The three-day death toll from the eastern offensive meanwhile stood at least 10 -- half of them servicemen -- as soldiers confronted gunmen in towns around Slavyansk.
AFP reporters near the eastern town of Kostyantynivka saw a pro-Russian checkpoint abandoned and smouldering while barricades were hastily erected in the centre.
Rebels defending Kostyantynivka told AFP there had been fighting overnight near the town's television tower.
In nearby Kramatorsk, pro-Russians were holed up in the town hall while burned-out trolley buses and minivans blocked off streets in the city centre.
But in the centre of besieged Slavyansk -- whose outskirts saw fierce gun battles on Saturday -- the situation remained relatively calm. Some of its 160,000 citizens reported increasing difficulty obtaining basic foodstuffs.
In annexed Crimea there were clashes between police and 2,000 pro-Kiev Tatars demonstrating against Russia's refusal to allow their leader Mustafa Dzhemilev into the peninsula.
The spreading violence eclipsed the small nugget of positive news in Ukraine on Saturday: the release of seven European OSCE inspectors, who were all safely home after a Russian envoy went to Slavyansk to organise their release.
Ukraine's violence sparked a new round of accusations and counter-accusations between the United States and Moscow as relations between the Cold War foes continued to suffer.
Russian Foreign Minister Sergei Lavrov called his US counterpart John Kerry to demand Washington use its influence over Kiev to stop what he called Ukraine's "war against its own people".
Lavrov warned that the military operations were pushing the former Soviet republic towards a "fratricidal conflict" and urged a greater mediating role for the OSCE.
Moscow has pronounced dead an accord struck last month in Geneva to defuse the crisis.
And it has dismissed Ukraine's plans for a presidential election on May 25 as "absurd" given the country's spiral into conflict.
Its stance has opened up the possibility that the West could impose its toughest punishment yet on Russia over the crisis.
US President Barack Obama said he would impose broader sanctions against Moscow if it destabilised its neighbour ahead of the election.
Kerry stressed to Lavrov the "possibility or the reality of sectoral sanctions" targeting specific areas of the already weakening Russian economy.
He hailed the release of the OSCE inspectors as a welcome step, but stressed that others needed to be taken "to de-escalate the situation".
The separatists in Ukraine were preparing their own spoiler of the May 25 election by moving ahead with plans to hold an independence referendum next Sunday.
The presidential vote was called by Ukraine's new leaders shortly after the ouster of pro-Russian leader Viktor Yanukovych in February, the culmination of months of pro-EU protests.