Heavy fighting is reported in and around Kramatorsk in eastern Ukraine.
Government forces appear to have surrounded the town and say they have reclaimed some buildings in the centre from the rebels.
Kramatorsk is near the rebel city of Slaviansk where two Ukrainian helicopters were shot down on Friday.
"We are not stopping," Interior Minister Arsen Avakov said in a post on Facebook. "The active phase of the operation continued at dawn."
Kiev said most of the dead who had been identified so far were foreign demonstrators from Transdniestria, a breakaway region of Moldova where Russia has a military garrison.
At least 37 people earlier died in a blaze in a trade union building in the port of Odessa.
On Saturday, police raised the overall death toll in the city to 42. It was the biggest death toll since about 100 people were killed in Kiev protests that toppled President Viktor Yanukovich in February.
Rebels in Donetsk aim to hold a referendum on 11 May on secession from Ukraine, similar to the vote on 16 March in the Crimea region, which was later annexed by Russia