The first group of Russian war planes has left Russia's Hmeimim air base in Syria, the defence ministry said.

The group included Sukhoi-34 fighter-bombers.

The ministry said each group will be led home by either a Tupolev-154 passenger plane or an Ilyushin-76 transport aircraft carrying engineers, technical personnel, and cargo.

President Vladimir Putin unexpectedly announced yesterday that "the main part" of Russian armed forces in Syria would start to withdraw, telling his diplomats to step up the push for peace as UN-mediated talks resumed on ending the five-year-old war.

Syria rejected any suggestion of a rift with Russia, saying President Bashar al-Assad had agreed on the "reduction" of Russian forces in a telephone call with Mr Putin.

Russia's military intervention in Syria in September helped to turn the tide of war in President Assad's favour after months of gains in western Syria by rebel fighters, who were aided by foreign military supplies including US-made anti-tank missiles.

Mr Putin made his surprise announcement, that came with no advance word to the United States, at a meeting with his defence and foreign ministers.

Russian forces had largely fulfilled their objectives in Syria, Mr Putin said.

But he gave no deadline for the completion of the withdrawal and said forces would remain at a seaport and airbase in Syria's Latakia province.

Meanwhile, United Nations human rights investigators have said that preparing prosecutions against war criminals should not wait for the conflict to end.

The chair of the Commission of Inquiry on Syria told the UN Human Rights Council that "the adoption of measures that lay the ground for accountability need not and should not wait for a final peace agreement to be reached."

He urged the Syrian government and opposition parties, who are taking part in peace talks in Geneva, to agree to confidence building measures, including the unconditional and immediate release of all prisoners arbitrarily detained.

He aldo called for the establishment of a mechanism to trace missing people.