NOVOCHERKASSK, Russia — The convoy of cars crawled along a potholed road deep in the flatlands of separatist-controlled eastern Ukraine on what was supposed to be a routine trip for a rebel leader. Instead, it ended in disaster.

First, a bomb exploded on the roadside. Then, machine-gunners opened fire on the immobilized convoy. When the ambush was over, five bodyguards, a press secretary and Aleksei B. Mozgovoi, the rebel leader, lay dead.

Rebels have been fighting in Ukraine for more than a year now, but the bloody assault this May was different: a massacre carried out by separatist rebels against another rebel group and former ally, the famed Cossacks, in a clash over the groups’ competing territorial claims.

The bitter feuding between the groups raises the prospect of greater factional fighting among the rebel forces, deepening an already grave humanitarian situation and possibly complicating matters for President Vladimir V. Putin of Russia, whom the West accuses of arming and supporting the separatists.