A Ukrainian military spokesman says government forces have withdrawn from terminal buildings at Donetsk airport, positions that they fiercely defended for months against pro-Russian separatists.
Meanwhile, media reports say at least six and as many as 13 people were killed on January 22 by an explosion at a public transport stop in the rebel-held city of Donetsk.
The cause of the explosion was not immediately clear, but some reports said a mortar or artillery shell hit a trolleybus.
Ukraine's Defense Ministry said on January 22 that the southern part of Donetsk airport remained under the control of government forces and a battle was continuing there.
It said 20 Ukrainian soldiers withdrew from the ruins of the terminal buildings late on January 21 because "their positions had been destroyed and exposed to direct fire."
The statement said that least six government soldiers were killed in the fighting at the airport during the previous 24 hours along with four others killed by fighting elsewhere in eastern Ukraine.
It also said 16 government soldiers were wounded and captured by rebels at the airport, which is on the northern side of Donetsk.
Ukrainian military spokesman Vladyslav Seleznyov said the decision to withdraw from the terminal buildings was made late on January 21 because "it looks like a sieve and there's simply nowhere to hide there."
In Donetsk itself, in a neighborhood that is under the control of separatist fighters, monitors from the Organization for Security and Cooperation in Europe (OSCE) were investigating the public transport stop where an explosion struck a trolley bus on January 22.
A cameraman from the Reuters news agency said he saw six bodies on the ground near, and inside, the vehicle.
He said windows of nearby shops also were blown out by the explosion.
The city administration later said that seven people were killed, while statements by separatists and medics suggested a death toll as high as 13.
More than 4,800 people have been killed since April in the conflict between the separatists and Ukrainian government forces.
The latest carnage heightened the challenges faced by diplomats seeking to end the fighting.
It came hours after a meeting in Berlin that German Foreign Minister Frank-Walter Steinmeier said produced an agreement for the rebels and government forces to pull back heavy weaponry 15 kilometers from a demarcation line established under a cease-fire deal agreed in Minsk in September.
Steinmeier said the "contact group" -- consisting of representatives of Ukraine, Russia, and the OSCE -- would be meeting in the coming days to work out the details and timeline for the weapons pullback.
He said the agreement represented progress but "no breakthrough" in efforts to end the conflict, which has driven ties between Moscow and the West to a state of tension unseen since the Cold War.
Russia and the rebels have frequently accused Ukrainian forces of shelling civilian areas, while Kyiv and international organizations say the separatists are provoking a response by firing artillery from civilian areas.
The Interfax news agency quoted a Donetsk separatist leader, Denis Pushilin, as saying on January 22 that "any strike launched against civilian areas and civilian casualties will only serve to complicate the negotiating process."
There was no immediate comment from the Ukrainian government about the January 22 bus stop explosion in Donetsk.