DONETSK, Ukraine — The foreign ministers of Russia, Poland and Germany on Tuesday signaled progress toward a cease-fire in Ukraine, even as up to 40 separatists were reported to have been killed in a fierce battle for control of an airport in the east of the country.

Talks aimed at a diplomatic resolution to the unrest have achieved some progress in recent days, the ministers said, but no firm agreement. Past efforts to broker a truce, including one by European and American officials, failed because separatists insisted that Russia did not speak for them at the talks.

Exactly who does speak for them is unclear. On Tuesday, the militant wing of a rebel group in Slovyansk, an epicenter of the violence, said it had arrested the “people’s mayor” of the city, Vyacheslav Ponomarev, who had been the public face of the pro-Russian uprising there, a Russian television channel, LifeNews, reported.

Authority had already been shifting to a Russian national, a militant who goes by the nickname Strelok, or “The Shooter.” It was unclear whether the shift was related to the cease-fire talks.