Hizbullah chief Sayyed Hassan Nasrallah has dismissed the reports about the presence of Hizbullah fighters in Ukraine as "mere rumors and lies."

Russia on Friday began withdrawing troops that had been running drills near the borders of Ukraine, the defence ministry said, following weeks of heightened tensions between Moscow and the West over the buildup.

Russia's FSB security service said on Saturday it had detained a Ukrainian diplomat, accusing him of receiving sensitive information from a Russian national.

French President Emmanuel Macron and German Chancellor Angela Merkel will hold talks on Friday with Ukrainian President Volodymyr Zelensky in a show of support for the former Soviet state, which fears a Russian invasion.

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.