Ukrainian forces on Thursday abandoned their defence of a long-disputed airport in the country's separatist east and vowed a response to Russia's escalating "aggression" in one of the deadliest days of the nine-month war.

In a graphic illustration of the worsening conflict, pro-Russian rebels also paraded some 20 captured Ukrainian soldiers through the city of Donetsk and forced them to kneel before enraged locals who threw snowballs and glass at them, some of it from a bus hit by shelling.

At least 44 people were reported killed on Thursday, including 13 civilians left dead in the trolleybus shelling in the rebel bastion city, the day's bloodiest incident.

A nine-year-old boy in the town of Maryinka near Donetsk was also killed, hit by shrapnel while having dinner with his family.

The violence came only hours after peace talks in Berlin called for a ceasefire and as the toll from the conflict topped 5,000 dead, with a million people also forced from their homes.

Another 10,000 have been wounded by rocket and mortar strikes that have been raining down on the industrial region's residential districts, Michael Bociurkiw of the Organisation for Security and Co-operation in Europe told Ukrainian radio.

After a flare-up in clashes at Donetsk airport in recent days, Ukraine's military said early Thursday its troops had abandoned most of the site.

The airport had become the symbolic prize of the conflict, with the army and rebels continuously battling for control.

Defence officials said fighting continued around the ruins of the air hub -- once one of eastern Europe's most modern and busiest -- but they admitted that government forces no longer controlled any of its major buildings.

On Thursday evening, President Petro Poroshenko called in his defence chiefs for an emergency meeting to plan a response to Russia's escalating "aggression", his office said.

"We have pulled up extra reserves, and if the enemy does not want to respect the ceasefire, if the enemy does not want to end the suffering of civilians... we will be ready to hit them in the teeth," Poroshenko's office quoted him as saying.

- 'A monstrous crime' -

The trolleybus shelling was among the deadliest incidents yet involving civilians in a conflict that has devastated the ex-Soviet republic's industrial heartland.

Stunned Donetsk residents gathered around the shredded remains of the bus, where the bloodied bodies of some elderly victims could still be seen sprawled in their seats hours after the attack.

An official with the city's emergency services said 12 people died in the bus blast while another was killed in a passing car.

Kiev and the rebels traded blame for the attack.

Ukrainian Prime Minister Arseniy Yatsenyuk accused the insurgents of committing "a horrible act against humanity."

"And responsiblity for this is borne by the Russian Federation," he added.

The rebels said Kiev's leaders had orchestrated the "barbaric" attack, while Russian Foreign Minister Sergei Lavrov called the bus shelling "a monstrous crime" whose ultimate responsiblity rested with "the party of war in Kiev and its foreign sponsors."

The strike occurred just south of Donetsk's city centre, far from the front line.

Later in the day, the Ukranian prisoners were paraded around the city, some of them injured, bandaged and limping, and were forced to kneel before angry residents for five to 10 minutes at the scene of the bus explosion.

- 'Russian occupation plan' -

Thursday's violence came hours 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.

German Foreign Minister Frank-Walter Steinmeier said the main achievement was that all sides had agreed that the demarcation line agreed in September would form the basis for the pull-back of heavy arms on both sides.

But US Secretary of State John Kerry accused the rebels of attempting "a blatant land grab", while Washington's UN envoy Samantha Power said Russia was pursuing an "occupation plan" in the east.

Poroshenko, speaking on Wednesday at the World Economic Forum in Davos, said the upsurge in fighting after a nearly month-long lull was prompted by a new surge of Russian forces and weapons.

"We have more than 9,000 troops of the Russian Federation on my territory, including more than 500 tanks and heavy artillery and armed personnel carriers," he said.

Moscow strongly denies supporting the insurgents despite NATO satellite imagery purporting to show its forces' presence in Ukraine.