Ukrainian troops and Moscow-backed separatists began a tense ceasefire Monday in a new bid to try and end years of hostilities in the ex-Soviet country's east.

Ukraine on Saturday demanded that Iran punish those guilty for the downing of a Ukrainian airliner and compensate victims while praising Tehran for cooperating with the "objective" investigation.

Iran said Saturday it "unintentionally" shot down a Ukrainian passenger jet, killing all 176 people aboard, in an abrupt about-turn after initially denying Western claims it was struck by a missile.

Iran's civil aviation chief Ali Abedzadeh said Friday he was "certain" a Ukrainian airliner which crashed outside Tehran this week was not hit by a missile.

A Ukrainian airliner crashed shortly after take-off from Tehran on Wednesday killing all 176 people on board, mainly Iranians and Canadians.

Search-and-rescue teams were combing through the smoking wreckage of the Boeing 737 flight from Tehran to Kiev but officials said there was no hope of finding anyone alive.

Moscow and Kiev on Monday signed a five-year agreement on the transit of Russian gas to Europe via Ukraine, after months of difficult talks but just ahead of a looming New Year deadline.

The current gas transit deal between the two ex-Soviet countries expires Tuesday and ties between them have been shredded since Moscow annexed Crimea in 2014 and supported a separatist insurgency in eastern Ukraine.

Ukraine on Thursday brushed off the impeachment of Donald Trump an a US "internal issue", despite the process being triggered by a telephone conversation between the American leader and his Ukrainian counterpart.

Trump was impeached on Wednesday by the House of Representatives for abuse of power over the July conversation with President Volodymyr Zelensky.

Monday's summit aiming to end the conflict in eastern Ukraine will see Russia's Vladimir Putin face off with Ukraine's Volodymyr Zelensky for the first time.

Fourteen people remain missing from a fire that claimed a student's life in Ukraine's southern port city of Odessa, President Volodymyr Zelensky said Thursday.

Russia said Monday it had handed over three navy ships it seized a year ago from Ukraine, in the latest move to ease tensions between the two countries ahead of a crucial summit.