"I can't keep them straight anymore," said a fighter who was buying walkie-talkies in preparation for what fighters here expect will be a major showdown with Ukraine's military.
The tangle of rebel groups presents a challenge for Ukraine as it struggles to quell a separatist movement inflaming its eastern edge, now in its fifth month. While the United States, the European Union and Russia would like to arrange a negotiated settlement, the fluidity and occasional hostility among the rebel groups is complicating the difficult task of getting peace talks off the ground.
The calculus may have changed recently with the appearance in Donetsk of Igor Girkin, a Russian citizen and rebel leader who goes by the name Strelkov, or shooter. He surfaced over the weekend along with hundreds of rebels who abandoned their positions after being overpowered by the Ukrainian military in weeks of fighting in the city of Slovyansk.
His presence raised the question of the role of Russia, Ukraine's powerful neighbor, whose next steps will be crucial to this country's fate. The West has accused Russia of arming the rebels, even while the Kremlin called for a peaceful resolution of the conflict.
But in recent weeks, fighters have complained that Moscow abandoned them, a sentiment that burst into public view this week when a political strategist closely allied with the Kremlin was shouted at by fighters at a news conference here.
The deputy prime minister of the Donetsk People's Republic, Andrei Purgin, said in an interview on Wednesday that Mr. Strelkov was trying to stitch the patchwork of militias into one professional army, to which soldiers would swear an oath. But the task is difficult, he said, as some fighters want to stick close to home.
"It is our hope that it will be one collective organ," Mr. Purgin said, sitting on a yellow couch in the government administration building in Donetsk that the rebels control.
As the rebel groups prepared for a fight in Donetsk, Ukraine's military continued to move troops and machinery into place around the city. Just south of Donetsk on Wednesday, Ukrainian forces steered a long column of more than 100 military vehicles toward the city. The column included a half dozen tanks, mobile launch rocket systems, armored personnel carriers and open-backed trucks crammed with troops.
Civilians filled the city's main train and bus stations, lining up for tickets out of town to escape the military campaign they feared was soon to come. The military bombed and shelled Slovyansk to force rebels from the city, and many here said they were afraid of what was in store for Donetsk.
"I'm not expecting anything positive to come out of this," said Vladimir, 30, a railroad worker, who was holding his 5-year-old daughter, Victoria, and said he had been waiting for train tickets to Kharkiv for more than four hours.
A Ukrainian military spokesman, Vladislav Seleznyov, said the rebels control just a third of the territory they once did before the government began fighting them some months ago. The claim could not be verified. The military is inching into the city of Luhansk, where it now controls an airport and a suburb called Metalist. Three people were killed in Luhansk in fighting this week.
The Luhansk region is poorer and grittier than that of Donetsk, and the rebel groups there are fractured. Michael Bociurkiw, a spokesman for the Organization for Security and Cooperation in Europe's special monitoring mission in Ukraine, said that inspection teams had observed different armed groups operating in the region that were not necessarily aligned or even in communication, raising another potentially serious obstacle to achieving a negotiated settlement.
He said the mission has seen "the existence of many small armed groups of individuals" in the region of Luhansk. "They are not necessarily connected to each other, and so hence any type of peace agreement needs to take into account the existence of this type of patchwork situation of armed groups," he added.
Sabrina Tavernise reported from Donetsk, and David M. Herszenhorn from Kiev, Ukraine, The New York Times.