The U.N. Security Council, including Russia, strongly condemned the "reprehensible" shelling Thursday of a trolleybus in east Ukraine and said those responsible should be brought to justice.

At least 13 mostly elderly people were killed when a shell hit the trolleybus in the pro-Russian rebel stronghold of Donetsk in an attack Moscow branded a crime against humanity. 

France on Friday called for an immediate ceasefire in Ukraine's war-torn east, calling the upsurge  in fighting barely hours after peace talks in Berlin "dreadful".

"I renew a call in the firmest manner for a ceasefire... and to return to what we had agreed on yesterday," French Foreign Minister Laurent Fabius told AFP on the sidelines of the World Economic Forum in Davos.

Ukraine's pro-Russian rebels on Thursday blamed the Western-backed authorities in Kiev for a trolleybus shelling in the separatist stronghold of Donetsk that killed 13 people during the morning commute.

The "foreign ministry" of the self-proclaimed Donetsk People's Republic said the attack "provides further evidence of the barbaric crimes being committed by the Kiev leadership" and called for an investigation by the United Nations and the Organization for Security and Co-operation in Europe (OSCE).

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 defense 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.

Ukrainian forces on Thursday abandoned their defense 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.

Ukraine has asked the IMF for a new and broader rescue package, and will seek to renegotiate its debt with bond holders, officials said on Wednesday as the war-torn country fights to save its reeling economy.

IMF chief Christine Lagarde met Ukrainian Petro Poroshenko on the sidelines of the World Economic Forum in Davos and said Ukraine requested that the Washington-based IMF deepen its relationship with Kiev.

The shelling of a bus and clashes in Ukraine's separatist east left 34 people dead Thursday as Kiev was forced to abandon its defense of Donetsk's airport in one of the deadliest days of the nine-month war.

In the worst incident, 13 civilians died when shelling hit a trolleybus Thursday morning in the rebel bastion of Donetsk, with Kiev alleging that ultimate blame for the tragedy rested with Russia.

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

The talks had been held against the unpromising backdrop of fresh clashes between Ukrainian government forces and pro-Moscow rebels in the east of the former Soviet republic, and after Ukraine's president accused Moscow of fueling the war with fresh troops and tanks.

U.S. Secretary of State John Kerry accused pro-Russian separatists Wednesday of attempting "a blatant land grab" in fresh fighting in eastern Ukraine.

The U.S. was "particularly concerned" by a rebel initiative to "attempt to gain control of a very significant rail juncture" in eastern Ukraine in violation of a September ceasefire deal, he said, calling it "an effort to try to broaden the amount of territory that is being held."

President Petro Poroshenko said on Wednesday more than 9,000 Russian troops were backing Ukrainian separatist fighters that Moscow will be under pressure to reel in at high-stakes peace talks in Berlin.

Poroshenko's claims followed days of heavy fighting that has left an already shaky September truce in tatters and forced the pro-Western leader to cut short his visit to the World Economic Forum in Davos.