DONETSK - At least 41 people were killed in Ukraine's east Thursday, one of the deadliest days in the separatist war, with a bloody bus shelling in Donetsk as government forces abandoned their defence of the city's strategic airport.
In a graphic illustration of the degenerating nine-month conflict, pro-Russian rebels also paraded some 20 captured Ukrainian soldiers through Donetsk and forced them to kneel before enraged locals who threw snowballs and glass at them, some of it from the shattered bus.
The trolleybus shelling in the rebel bastion city was the day's bloodiest incident, with 13 civilians killed and Kiev alleging that ultimate blame for the tragedy rested with Russia.
The violence came only hours after peace talks in Berlin called for a ceasefire and as the toll from the conflict surpassed 5,000 dead, with a million people also forced from their homes.
Another 10,000 have been wounded by rocket and mortar strikes raining down daily on the industrial region's residential districts, Michael Bociurkiw of the Organisation for Security and Co-operation in Europe told Ukrainian radio.
With intensifying clashes rocking Donetsk airport in recent days, Ukraine's military said early Thursday that 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 controlled little more than a few isolated buildings on its outskirts.
The trolleybus shelling was among the bloodiest incidents involving civilians in recent months in a conflict that has devastated the ex-Soviet republic's industrial heartland and brought Ukraine's economy to its knees.
Stunned Donetsk residents gathered around the shredded remains of the bus, with bloodied bodies of elderly victims sprawled in their seats hours after the attack and local militias cautiously inspecting the damage.
An official with the city's emergency services said 12 people died in the bus while another was killed in a passing car.
Russian Foreign Minister Sergei Lavrov called the attack "a monstrous crime" whose ultimate responsiblity rested with "'the party of war' in Kiev and its foreign sponsors."
Ukrainian Prime Minister Arseniy Yatsenyuk made similar charges against Moscow, accusing the insurgents of committing "a horrible act against humanity.".
"And responsiblity for this is borne by the Russian Federation," Yatsenyuk said.
The strike occurred just south of Donetsk's city centre far from the front line, with concerns having been raised over shelling that has at times moved closer in from the airport northwest of the city, putting civilians in increasing danger.
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 shelling.
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.
The talks had been held against the unpromising backdrop of fresh clashes and after Ukraine's president accused Moscow of fuelling the war with fresh troops and tanks.
The four top diplomats in their statement could agree to "call on all sides involved to cease hostilities and to withdraw heavy weapons" back from a demarcation line agreed in the widely flouted September truce signed in Minsk.
German Foreign Minister Frank-Walter Steinmeier said the main achievement was that all sides had agreed that the demarcation line agreed in the Minsk pact would form the basis for the pull-back of heavy arms on both sides.
The talks had "tested the patience of all participants", he said after meeting his Russian counterpart Sergei Lavrov, who left the talks first, as well as Ukraine's Pavlo Klimkin and Laurent Fabius of France.
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.
Ukrainian President Petro 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.