Kyiv says fighting has resumed near the town of Maryinka, 30 kilometers west of the rebel stronghold of Donetsk in eastern Ukraine.
Military spokesman Andriy Lysenko said on June 8 that pro-Russian separatists opened fire on government positions there using mortars and firearms.
A rebel defense official, Eduard Basurin, rejected the statement as "nonsense."
Ukraine's military said that a June 3 rebel offensive near Maryinka forced it to use artillery that had been removed under the cease-fire agreement signed in Minsk in February.
The Organization for Security and Cooperation in Europe's monitoring mission in eastern Ukraine has warned that violations of the truce deal are becoming more frequent and severe.
More than 6,400 people have been killed since fighting erupted in Ukraine's east between government forces and separatists in April 2014.