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. Prime Minister Arseniy Yatsenyuk said Russia was 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 under-attack police in the headquarters also released some of the 150 pro-Russian militants arrested in Friday's clashes. 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. - Fighting around rebel bastion - 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 Russian President Vladimir Putin says he has a right to launch -- but "hopes" he won't have to. Ukrainian officials have pushed on regardless with the operation, determined to crush the pro-Kremlin rebels. They have also put the 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 10 at least -- half of them servicemen -- as soldiers confronted gunmen in towns around the rebel bastion of 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 was relatively calm. Some of its 160,000 citizens reported increasing difficulty obtaining basic foodstuffs. Sporadic fighting was also reported overnight in the eastern city of Lugansk and the port city of Mariupol. 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. - 'Fratricidal conflict' - 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". As Moscow and Washington traded barbs about interference in Ukraine, Germany's Bild am Sonntag weekly alleged that dozens of US intelligence agents were advising the Kiev authorities, citing unnamed Germany security sources. 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.