Leonid Kravchuk, the head of the Ukrainian delegation to the Trilateral Contact Group in the Donbas region of eastern Ukraine (with the participation of Russia, the Ukrainian government and representatives of the opposition in the east) announced on Wednesday that he had written a letter to the OSCE chairman.
The agency has been called in to investigate ceasefire violations in the region and elsewhere in eastern Ukraine, he said.
"Since March 30, three Ukrainian soldiers have been killed and four others wounded in a shooting at a checkpoint in the Donbas region," Kravchuk wrote in the letter.
Tensions in Donbas between the opposition and the Ukrainian government have escalated since last Sunday.
Two days ago, a 5-year-old child was killed and another was injured when a Ukrainian army drone struck a residential area near the eastern city of Donetsk.
A total of nearly 14,000 people, mostly civilians, were killed during the civil war in eastern Ukraine between Ukrainian forces and separatists, which ended a year ago with a ceasefire between the two sides.