Ukrainian forces have raised their national flag over a police station in the city of Luhansk that was for months under rebel control, Kiev said on Sunday, in what could be a breakthrough in Ukraine's efforts to crush pro-Moscow separatists.
Ukrainian officials said, however, the rebels were fighting a desperate rearguard action to hold on to Luhansk - which is their supply route into neighboring Russia - and that the flow of weapons and fighters from Russia had accelerated.
The foreign ministers of Ukraine, Russia, France and Germany held a meeting in Berlin on the crisis, with German Foreign Minister Frank-Walter Steinmeier saying afterwards they would report back to their leaders and possibly agree on Monday or Tuesday on how to continue talks.
"The aim remains to bring about a ceasefire in Ukraine and to prevent future victims," Steinmeier said.
"It was a difficult discussion but I believe and I hope that we made progress on some points," he told reporters.
Russia denies helping the rebels and accuses Kiev, backed by the West, of triggering a humanitarian crisis through indiscriminate use of force against Russian speakers in eastern Ukraine who reject the Ukrainian government's rule.
Andriy Lysenko, a Ukrainian military spokesman, said government forces fought separatists in Luhansk on Saturday and took control of the Zhovtneviy neighborhood police station.
"They raised the state flag over it," Lysenko said.
Separatist officials in Luhansk could not be reached by telephone, and a separatist spokeswoman in Donetsk, the other rebel strong-hold, said she had no information about Luhansk.
A photograph posted on Twitter appeared to show a Ukrainian flag on the front of the police station, but it could not be independently verified.
If confirmed, the taking of the police station is significant because Luhansk has for several months been a rebel redoubt where Kiev's writ has not run. Separatists still control sections of the border linking Luhansk region to Russia.
Ukrainian President Petro Poroshenko announced another military success, saying his forces had recaptured a railway junction at Yasynuvata, north of Donetsk, which he said had "strategic significance".
The four-month-old conflict in Ukraine's Russian-speaking east has reached a critical phase, with Kiev and Western governments watching nervously to see if Russia will intervene in support of the increasingly besieged rebels.
The rebels responded with defiant rhetoric and fighting.
Ukrainian authorities said on Sunday the separatists shot down a Ukrainian warplane.