Ukraine says five of its soldiers were killed and 39 wounded on June 3 in fighting between government forces and Russian-backed separatists in the country's east.
Presidential aide Yuriy Biryukov said on Facebook on June 4 that most casualties were reported near the town of Maryinka, west of the rebel stronghold of Donetsk.
Kyiv says the separatists, using tanks and artillery, tried to take government positions there, in violation of a tenuous internationally brokered peace deal.
Ukraine's military says the offensive forced it to use artillery that had been removed under the Minsk II agreement as part of an effort to create a demilitarized zone.
The deal has served as the basis for a shaky cease-fire since February.
The rebels blamed government troops for the fighting and said 15 civilians and separatist fighters died in shelling by government forces.
In Moscow, Russian Foreign Minister Sergei Lavrov said the Minsk agreement was "constantly under threat because of the actions of the Kyiv authorities."
Based on reporting by Reuters and Interfax