The United Nations on Friday urged all sides in Ukraine to fully uphold a Europe-brokered ceasefire, as the warring parties accused each other of jeopardizing the truce.

U.N. political chief Jeffrey Feltman warned during an emergency Security Council meeting: "We are either looking at a return to a deepening intractable conflict or a momentary upsurge in parts of the conflict zone.

Ukrainian President Petro Poroshenko warned Thursday of the "colossal threat" of major clashes in eastern Ukraine where at least 26 people have died in renewed fighting between government forces and pro-Russian rebels.

Fresh clashes erupted near the rebel stronghold of Donetsk on Wednesday, in one of the worst outbreaks of violence since a European-brokered truce was agreed in February.

Fighting surged in east Ukraine on Wednesday as Kiev accused pro-Russian forces of launching a "large-scale" attack in breach of a tenuous ceasefire. 

Rebels claimed that 15 separatist fighters and civilians were killed as fresh clashes flared, but denied that they had started a new assault.  

Kiev on Tuesday accused Moscow of sabotaging Ukraine crisis talks, after a meeting in Minsk aimed at resolving the conflict in eastern Ukraine was postponed.

Officials from Ukraine, Russia and the Organization for Security and Cooperation in Europe (OSCE) mediator group decided to postpone the meeting with representatives of pro-Russian separatists, OSCE representative Heidi Tagliavini told journalists.

The Russian firm making missiles similar to the one the West claims downed Malaysia Airlines flight MH17 in Ukraine said on Tuesday the passenger jet was likely hit by a BUK missile system. 

All 298 passengers and crew on board the Malaysia Airlines jetliner -- the majority of them Dutch -- died when it was shot down over rebel-held eastern Ukraine last year.

The U.S. military released a video Monday of a Russian Su-24 bomber flying past an American warship in the Black Sea to dispel what it called inaccurate media reports about a routine encounter.

In the video, a Su-24 aircraft appears in the distance, then zooms by the USS Ross -- a guided-missile destroyer. 

Russian Foreign Minister Sergei Lavrov on Monday dismissed the "absurdity" of EU complaints over a travel ban imposed by Moscow on 89 Europeans over the Ukraine crisis.

"It is even quite embarrassing to explain the absurdity and awkwardness of such logic as it's an attempt to substitute international law with one's own political biases," Lavrov told reporters in Moscow amid a new spike in tensions with Brussels.

The United Nations said Monday that more than 6,400 people had been killed in conflict-wracked Ukraine, and despite a slowdown in fighting, millions more are suffering from abuses and hardship.

The U.N. human rights office also said there was increasing evidence that Russian servicemen are taking part in the hostilities.

Three Ukrainian soldiers and two civilians have been killed over the past 24 hours in fighting in the separatist east, a Ukrainian military spokesman said Sunday.

The latest deaths highlight the fragile nature of a February ceasefire aimed at halting the conflict and come shortly before talks next week aimed at salvaging the deal.

European politicians who discovered they were on a previously confidential Russian travel blacklist said Saturday they were proud of being included, as EU governments led by Germany criticized the entry bans.

The 89-strong list, formerly undisclosed but revealed to European diplomats on Thursday, includes past and serving parliamentarians and ministers who have been outspoken critics of President Vladimir Putin and the war in Ukraine.