Russian President Vladimir Putin unexpectedly announced today that "the main part" ofRussian armed forces in Syria will start to withdraw.
He told 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.
Western diplomats speculated that Mr Putin may be trying to press Mr Assad into accepting a political settlement to the war, which has killed 250,000 people.
US officials note they have seen 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 Mr 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, he 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 road map" for Syria failed to make progress.
Mr Putin and US President Barack Obama spoke by phone today 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 Mr 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 the US was encouraged by Mr 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.
Mr Putin said at the Kremlin meeting that he was ordering the withdrawal from tomorrow 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.
Kremlin spokesman Dmitry Peskov said Mr Putin had telephoned the Syrian president to inform him of the decision, but the two leaders had not discussed Mr Assad's future, the biggest obstacle to reaching a peace agreement.
The move was announced on the day UN-brokered talks involving the warring sides in Syria resumed in Geneva.
In Damascus, the Syrian presidency said in a statement that Mr Assad had agreed to the reduction in the Russian air force presence, and denied suggestions it reflected a difference between the two countries.
"The whole subject happened in complete coordination between the Russian and Syrian sides, and is a step that was carefully and accurately studied for some time," the statement said, adding that Russia had promised to continue support for Syria in "confronting terrorism".
Syria regards all rebel groups fighting Mr Assad as terrorists.
Rebels and opposition officials alike reacted sceptically.
"I don't understand the Russian announcement, it's a surprise, like the way they entered the war. God protect us," said Fadi Ahmad, spokesman for the First Coastal Division, a Free Syria Army group fighting in the northwest.
Opposition spokesman Salim al-Muslat demanded a total Russian withdrawal. "Nobody knows what is in Putin's mind, but the point is he has no right to be in be our country in the first place. Just go," he said.
A European diplomat was also sceptical. "It has the potential to put a lot of pressure on Assad and the timing fits that," the diplomat said.
"However, I say potentially because we've seen before with Russia that what's promised isn't always what happens."
2.4 million child refugees
Meanwhile Syria's five-year conflict has created 2.4 million child refugees, killed many and led to the recruitment of children as fighters, some as young as seven, UNICEF has said.
Its report "No Place for Children" says more than eight million children in Syria and neighbouring countries need humanitarian assistance, with the international response plan for Syria chronically underfunded.
"Twice as many people now live under siege or in hard-to-reach areas compared with 2013. At least two million of those cut off from assistance are children, including more than 200,000 in areas under siege," it says.
The UN says more than 450,000 people are under siege.
Cases of starvation have been reported this year in areas surrounded by government forces and their allies near Damascus, and by the so-called Islamic State in eastern Syria.
Violence continues despite a fragile cessation of hostilities reached last month.
UNICEF has said 400 children were killed in 2015.
A separate report on Friday by a number of aid groups, including Oxfam, said UN figures showed at least 50,000 people had been killed since April 2014.
Outside Syria, more than 151,000 Syrian children have been born as refugees, according to UNICEF.
UN refugee agency UNHCR says nearly 70,000 Syrian refugee children have been born in Lebanon alone.
About 2.8 million Syrian children in Syria or neighbouring countries are not attending school. Dozens of schools and hospitals were attacked in 2015, according to aid groups.