Russian President Vladimir Putin meets Foreign Minister Sergei Lavrov at the Kremlin in Moscow, Russia, March 14, 2016.[Photo/Agencies]

MOSCOW/GENEVA - President Vladimir Putin announced out of the blue on Monday 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.

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

Western diplomats speculated that Putin may be trying to press Assad into accepting a political settlement to the war, which has killed 250,000 people, although US officials saw no sign yet of Russian forces preparing to pull out.

The anti-Assad opposition expressed bafflement, with a spokesman saying, "Nobody knows what is in Putin's mind".

Russia's military intervention in Syria in September helped to turn the tide of war in 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.

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, 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.

In Geneva, United Nations mediator Staffan de Mistura told the warring parties there was no "Plan B" other than a resumption of conflict if the first of three rounds of talks that aim to agree a "clear roadmap" for Syria failed to make progress.

Putin and US President Barack Obama spoke by phone on Monday about Syria, with the Kremlin saying the two leaders "called for an intensification of the process for a political settlement" to the conflict.

The White House said Obama welcomed the reduction in violence since the beginning of the cessation of hostilities but "underscored that a political transition is required to end the violence in Syria."

A US official, speaking on condition of anonymity, said Washington was encouraged by Putin's announcement but that it was too early to say what it means, whether he will carry it out and what may have motivated it.

Putin said at the Kremlin meeting that he was ordering the withdrawal from Tuesday of "the main part of our military contingent" from the country.

"The effective work of our military created the conditions for the start of the peace process," he said. "I believe that the task put before the defence ministry and Russian armed forces has, on the whole, been fulfilled."

With the participation of the Russian military, Syrian armed forces "have been able to achieve a fundamental turnaround in the fight against international terrorism", he added.