“They tell me my job for now is to eat and get stronger,” he said.

What once seemed a sideshow to Ukraine’s campaign against the rebel militias took center stage on Friday when a firefight pitting the Donbass Battalion against a pro-Russian militia left at least seven people dead just south of Donetsk, the largest city in the region.

The origins of the clash were in dispute.

Before the shooting had stopped, Semyon Semenchenko, the battalion’s leader, claimed that his fighters had been ambushed near a rebel checkpoint in Karlovka, a village about 45 minutes’ drive from Donetsk. Locals and pro-Russian fighters, however, said that Mr. Semenchenko had led an assault on the checkpoint and found himself surrounded when enemy reinforcements arrived.

The battle showed the devastating potential for violence between ad hoc militias, some with little formal training, which could continue even if clashes between the Ukrainian Army and the rebels cease. Until Friday, the Donbass Battalion had seen little action, and was known best for storming a police station in the small town of Velyka Novosilka. Yet the levels of violence have escalated recently. As many as 15 Ukrainian conscript soldiers were killed in an ambush on Thursday at a checkpoint on an isolated country road about an hour south of Donetsk.

Hundreds of Kiev supporters have joined the newly reinstated National Guard, hastily formed military units of inexperienced fighters who have found themselves on the front lines of the simmering conflict.