The unrest in the southern Ukrainian city of Odessa has flared anew with pro-Russian militants attacking a police headquarters.
Unrest in Ukraine has spread to the south as thousands of pro-Russian protesters attacked Odessa's police headquarters after a fire killed dozens of their comrades, in violence Kiev charged was a Russian plot to "destroy" the country.
Increasing tension 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.
A blaze at a trade union building in Odessa on Friday left 42 people dead - most of them pro-Russian militants - after running battles with pro-Kiev protesters.
On Sunday, police released 67 separatist militants they had detained following the violence in a bid to calm thousands of pro-Moscow supporters who had gathered outside Odessa's police headquarters. One person, though, was reported wounded by gunfire in the city.
Three loud explosions were also heard by AFP journalists after 8.00pm on Sunday (0300 AEST Monday), but a spokeswoman for the rebels said "the fighting was over".
Ukraine's Prime Minister Arseniy Yatsenyuk, who was in Odessa to observe mourning for the 42 dead, accused Russia of executing a plan "to destroy Ukraine and its statehood".
Several thousand supporters of a united Ukraine, some of them masked and carrying sticks, rallied in Odessa late on Sunday before marching to the regional police headquarters.
Demonstrators shouted "Odessa in Ukraine!" and "Glory to Ukraine", also singing the national anthem.
They then peacefully marched to the trade union building to re-deploy the Ukrainian flag that had been burned by supporters of closer ties with Russia on Saturday.
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. Instead it blames the Kiev government and its Western backers.
In a potentially significant diplomatic move late on Sunday, 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 spreading violence eclipsed Saturday's release of seven European OSCE inspectors who had been held for more than a week by pro-Russia militants in their eastern bastion of Slavyansk.
The Burkhalter 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.
Moscow has 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 will not have to.
But Ukrainian officials have pushed on with the operation, determined to crush the pro-Kremlin rebels.
Late on Sunday, a spokeswoman for the rebels in Slavyansk said "the town is completely surrounded".
AFP staff saw seven armoured vehicles blocking the last main road out of Slavyansk 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.
In the centre of Slavyansk, the situation remained relatively calm, although some of its 160,000 citizens reported increasing difficulty obtaining basic foodstuffs.
Ukraine's violence sparked a new round of recriminations between Washington and Moscow.
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.
US President Barack Obama said he would impose broader sanctions against Moscow if it destabilised its neighbour ahead of the election.