At least 13 people have been killed after a trolleybus was shelled in eastern Ukraine's Donetsk, an official said, hours after calls for ceasefire at a meeting in Berlin.

"Twelve people were killed in the trolleybus and another who was in a car that was passing nearby," said an official with the emergency services in the city, a bastion of pro-Russian rebels who are fighting Ukraine's military.

Around a dozen people were also wounded, the official said. It was unclear who was responsible for the attack.

However, pro-Russian separatists seemed to have putting the blame on Ukrainian soldiers. Shortly after the attack, the rebels paraded around 20 captive Ukrainian soldiers throught the streets of Donetsk. Dozens of the city's residents hurled snow and bits of glass from the shelling. 

The prisoners dressed in civilian clothes, some of them injured, bandaged and limping, faced the crowd after being forced to kneel.

"They have to be punised, like Saddam Hussein. They are killers. They killed our children," pensioner and local resident Zina told the AFP.

Another bystander expressed sympathy for the soldiers, however. "I am ashamed of what is happening," said Lyuda, also retired. 

"I have pity for them. Theya are victims of [President Petro] Poroshenko. They are not guilty, they were sent here," she said, looking at the captive soldiers with tears in her eyes.

Kiev argues the strike was committed by the separatists.

Army withdraws

Amid the fresh fighting, Ukraine's military said on Thursday that it had withdrawn its forces from the main part of a disputed airport in rebel-held Donetsk after months of bloody fighting.

"Yesterday evening, we made the decision to leave the new terminal," military spokesman Vladyslav Seleznyov told AFP news agency, in reference to the main airport building that had been held by Kiev forces since late May.

"On the whole, 10 soldiers were killed and 16 wounded in the past day of fighting" across the pro-Russian separatist east, he said.

Once the pride of the city but now reduced to a burned-out shell, the terminal is of limited strategic value. But it has since acquired a symbolic value because of the Ukrainian forces' stand against waves of separatist attacks.

Russian television reporters on Wednesday toured the airport building and showed the bodies of killed Ukrainian soldiers.

Demarcation line

The latest violence comes a day after the foreign ministers of Russia, Ukraine, Germany and France ended a crisis meeting in Berlin with a joint call to cease hostilities, but no breakthrough agreement to stop the bloodshed.

Under the plan agreed on Wednesday, Ukraine and the pro-Russian separatists would pull back their heavy arms 15 km on either side of the demarcation line defined in the Minsk agreement of last year, though there was no agreement on a withdrawal of all troops.

Earlier on Wednesday, Russian Foreign Minister Sergey Lavrov had urged measures to contain the unfolding unrest, but said nothing about the rebels surrendering territory they acquired in violation of a peace deal concluded in September in Minsk, Belarus.

Fighting between Ukrainian government troops and separatist forces surged after the New Year following a month of relative tranquility after a truce was declared in early December.

The conflict has killed more than 4,800 people since erupting in April 2014.

Source: Al Jazeera and agencies