Russia plans to bring together members of the Syrian regime and opposition in a new push to resuscitate talks on ending the country's nearly four-year civil war, although hopes of a breakthrough are slim.

The January 26-29 behind closed doors talks will include Syria's regime-tolerated domestic opposition and members of Bashar Assad's government but not the Western-backed exiled National Coalition.

U.S. spies intercepted communications between the chief suspects in the murder case of Russian former spy Alexander Litvinenko, linking his poisoning to the Russian state, Britain's Daily Telegraph reported Saturday.

According to the report, the National Security Agency (NSA) obtained electronic messages sent between London and Moscow shortly after the Kremlin critic was poisoned with radioactive polonium-210 while drinking tea at a hotel in the British capital in 2006.

The European Union called Friday on Moscow to "assume its responsibility" in ending the separatist war in Ukraine.

"Time is running out in eastern Ukraine where the escalation of fighting has caused far too many civilian as well as military casualties," EU foreign policy chief Federica Mogherini said in a statement.

Russian President Vladimir Putin on Friday accused the Ukraine government of ordering a major new offensive against pro-Russian rebels in the separatist east that has claimed dozens of lives in the past few days.

"Kiev authorities have given an official order to begin large-scale combat operations" along almost the entire frontline with pro-Russian separatists, Putin said in a meeting with his security council.

Pro-Russian rebels on Friday vowed to conquer more territory in eastern Ukraine and ruled out peace talks after Kiev retreated from a long-disputed airport, casting aside Europe's latest push for a truce.

The defiant comments from Donetsk separatist leader Alexander Zakharchenko came as Ukraine renewed allegations of Russian army units fighting with rebels across the frontline dividing the war-torn country's industrial east.

Pro-Russian rebels cemented their hold Friday on a long-disputed airport ceded by Ukrainian troops during an upsurge in clashes that killed nearly 50 people and punctured Europe's latest push for peace in the nine-month war.

The deadliest day of fighting since the signing of an increasingly irrelevant September truce also saw Moscow and Kiev on Thursday trade bitter blame for a trolleybus shelling in the rebel stronghold of Donetsk that killed 13 mostly elderly passengers.

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.

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.