DONETSK, Ukraine — The Ukrainian military on Sunday moved into the heart of the separatist hub of Luhansk for the first time, officials said, chipping at one of the cornerstones of the pro-Russia rebels’ disintegrating mini-state.
Ukrainian officials said army units had raised the national flag over a police station in central Luhansk, the eastern city that, along with Donetsk, has been a center of rebel activity and an important destination for Russian fighters and aid. Other parts of Luhansk, however, were said to still be under rebel control.
The claim could not be independently confirmed, though a photograph of the flag and police station was circulating on social media, and the report was consistent with the progress of fighting there.
Along with increased Ukrainian pressure on rebel positions in Donetsk, the army’s move into Luhansk focused attention on the profound reversal of the separatists’ fortunes since they declared independence in April.
Interviews across eastern Ukraine portray a rapid breakdown in discipline in the rebel ranks. Many fighters have abandoned their uniforms in favor of civilian clothes, and three senior leaders have left the war zone in recent days.
Even as Ukrainian officials in Kiev celebrated the developments, and as international foreign ministers met in Berlin to try to ease the crisis with Russia, there was increased worry that the Kremlin would take the rebels’ disintegration as an incentive to intervene more forcefully in Ukraine, perhaps sparking a wider war.
“It’s a case of being very, very, very careful what you wish for,” said Cliff Kupchan, an analyst with the Washington-based Eurasia Group, speaking of Western officials who have endorsed the Ukrainian military option. If President Vladimir Putin of Russia believes “the rebels are about to get routed, we do have a problem,” Kupchan said.
A large Russian convoy said to be carrying humanitarian aid to Luhansk remained at the Ukrainian border Sunday, and movements there were being closely watched. Western officials have warned that pushing the convoy through would be taken as a belligerent act by Russia.
In Berlin, the foreign ministers of Ukraine, Russia, France, and Germany met to discuss a settlement to the four-month-old conflict, which Russia stoked after a pro-Western government took power in Kiev after weeks of street protests.
In a telephone call with the Ukrainian president, Petro Poroshenko, Chancellor Angela Merkel of Germany said Saturday that Russia should clarify a statement by the separatist prime minister in Donetsk, Alexander Zakharchenko, who said he had received reinforcements from Russia. Russia has denied sending soldiers or weapons into eastern Ukraine.
Zakharchenko said rebels would receive 150 armored vehicles and 1,200 soldiers who, he said, had spent four months training in Russia. “They are joining at the most crucial moment,” Zakharchenko said.
In a radio interview Sunday, a spokesman for Putin denied Zakharchenko’s claim that a new supply of arms and fighters had been brought across the border from Russia.
“We have said repeatedly that no equipment is being supplied there,” the spokesman, Dmitry S. Peskov, said in the interview on the Moscow Speaks radio station.
While the Kremlin has steadfastly denied supplying the separatists in eastern Ukraine, insurgent leaders on several occasions have spoken openly about support from Russia and at times have complained about the quality of weapons and ammunition they received.
Besides the advance in Luhansk, the Ukrainian military made another critical move, capturing the town of Yasinovataya, a strategic railway hub north of Donetsk.
The army has tightened its siege on Donetsk in recent weeks, and inside, a breakdown in discipline among the rebel fighters had locals on edge.
Residents said rebel fighters had taken to drinking at night and wearing civilian clothes at checkpoints, a change of clothes that would allow them, at a moment’s notice, to throw down their weapons and blend in with the population.
In recent weeks, officials have spoken about the possibility of creating a corridor that would allow insurgent fighters who had arrived from Russia to put down their weapons and retreat back across the border.