Ukraine's armed forces said on Friday that four soldiers were killed in shelling by Russian forces in the eastern Donbass region, the highest daily death total since a ceasefire was agreed in July last year.
Ukraine and Russia have traded blame for a spike in violence in the conflict in eastern Ukraine, which Kyiv says has killed 14,000 people since 2014.
A Ukrainian armed forces statement said Russian forces had "once again violated the ceasefire and targeted the positions of Ukrainian defenders with 82-mm mortars, automatic grenade launchers and large-caliber machine guns prohibited by the Minsk agreements near the settlement Shumy in the area North."
It said four servicemen were killed and two injured.
Ukraine, Western countries and NATO accuse Russia of sending troops and heavy weapons to prop up separatists. Moscow says it only provides political and humanitarian support to rebels and says Russians fighting in Ukraine are volunteers.
Russian-backed forces seized a swathe of eastern Ukraine in 2014, including the industrial cities of Donetsk and Luhansk.
Major combat ended with a truce agreed in the Belarusian capital Minsk in 2015, whose implementation France and Germany have helped to oversee. But sporadic fighting continues despite repeated attempts to implement a ceasefire.