At least 42 Ukrainian troops were also wounded in confrontations with insurgents during the government's military operation outside Slavyansk, Vladislav Seleznyov, a spokesman for the Ukrainian forces, told reporters.
On the insurgents' site, some 300 people were either killed or injured, Seleznyov said.
Earlier in the day, Interior Minister Arsen Avakov said government forces were launching the "active offensive phase" of an "anti-terror" operation near Slavyansk. He urged civilians to stay at home.
Some 180 people, including 59 Ukrainian soldiers, have been killed since the beginning of the separatist uprising in eastern Ukraine in April, according to the country's Prosecutor General Office.