At least 31 people are reported to have died after a building in the Ukrainian city of Odessa was set on fire during clashes between Pro-Russian and pro-Kiev protesters.
Police said people inside the trade union building were overwhelmed by smoke while others were killed as they jumped from windows to escape the burning building.
Several people were seen balancing on an upper-storey ledge during the fire waiting to be rescued from the blaze.
The interior ministry could not confirm how the fire was started but reports suggest that separatists in the rebel-controlled region had barricaded themselves inside the building and both sides were throwing petrol bombs.
Ukrainian Deputy Foreign Minister Danylo Lubkivsky told the BBC he regretted the loss of life but insisted Russia was behind the violence.
"The situation remains under control. But the security situation is threatened by Russian special agents," he said. "That's why we are requesting and demanding Russian authorities to stop their efforts that can undermine regional security and stability."
The incident comes as the Kiev government launched a major offensive against pro-Russian activists who earlier seized government buildings in the east of the country.
Earlier President Oleksandr Turchynov said many separatists had been killed in a government offensive as the Ukrainian army took control of checkpoints around Slovyansk.
Russia said Kiev's offensive against the insurgents had "destroyed" the two-week-old Geneva agreement on cooling Ukraine's crisis.
Kiev claims it neighbouring country is preparing to invade and that it is stoking the unrest in the east.
Moscow denies the allegations, but has warned Russia would respond to attacks on Russian citizens or interests in the east, where insurgents have seized government buildings.
Latest reports from Slovyansk say intense gunfire has also broken out in the centre of the city.
The current death toll in Odessa is the highest since violence broke out between pro-EU protesters and police in the capital in February.