Fighting between Russian-backed rebels and Ukrainian government forces since April 2014 has resulted in thousands of deaths, the displacement of over a million people and de facto Russian control of the region. But feuding between proxy forces on the Russian side has for more than a year threatened to spiral into wider violence, deepening an already grave humanitarian situation and possibly complicating matters for President Vladimir V. Putin of Russia in negotiations with the West.

When the rebellion erupted in 2014, the Luhansk region was principally controlled by Don Cossacks, the onetime horsemen of the southern steppe, who have fought on the Russian side but have also tried to carve out their own republics from the chaos in Ukraine. The Cossacks are loyal to Russia but have for centuries dreamed of forming an autonomous state.

Cossack republics were declared in the Luhansk region, in three small towns, where traditional Cossack rule including punishment by public whipping was established. Starting in the spring of 2015, Mr. Plotnitsky had tried to bring them to heel, demanding that the Cossack units incorporate into the Russian-controlled rebel military.

Two Cossack leaders, Aleksei B. Mozgovoi and Pavel L. Dryomov, were killed in car bombings in 2015. Mr. Dryomov had posted a video on YouTube, addressed to Mr. Putin, that criticized Mr. Plotnitsky for trading coal with the enemy in government-controlled Ukraine and demanded his ouster.

Estimates of the number of casualties in the fighting between the Don Cossacks and the Luhansk People’s Republic forces range from 100, by the Cossacks’ count, to 200, according to Ukrainian law enforcement officials.