In a surprise move, Russian President Vladimir Putin has ordered his military to start withdrawing the "main part" of its forces in Syria from Tuesday.
He said the Russian intervention had largely achieved its objectives.
The comments come amid fresh peace talks in Geneva aimed at resolving the five-year Syrian conflict.
Russia is a key ally of Syrian President Bashar al-Assad, whose office said in a statement he had agreed to the move.
The pullout was "in accordance with the situation on the ground", the statement said.
Russia began its campaign of air strikes in Syria last September, tipping the balance in favour of the Syrian government and allowing it to recapture territory from rebels.
"I consider the mission set for the defence ministry and the armed forces on the whole has been accomplished," Mr Putin said in a meeting at the Kremlin.
"I am therefore ordering the defence ministry to begin the withdrawal of the main part of our military force from the Syrian Arab Republic from tomorrow."
Putin may be hedging his bets: Sarah Rainsford, BBC Moscow correspondent
When Russian air strikes began in Syria, President Assad's regime was on the brink of collapse. Less than six months later, Russia says its action allowed Syrian government troops to retake 10,000 sq km (3,860 sq miles) of territory.
By intervening, Vladimir Putin made clear that Russia was prepared to assert its interests. The results ensure Moscow a bigger say in what happens at the peace talks.
The decision to scale down Russian operations may partly be fuelled by cost, given falling oil prices; it could also be driven by a desire to end Russia's isolation and Western sanctions.
But whilst Vladimir Putin has ordered his foreign minister to focus efforts on the political front in Syria, he appears to be hedging his bets.
Critically, sophisticated air defence systems seem set to stay. And as we have never been told officially how many troops were ever sent to Syria, we are unlikely to know how many will remain.
Mr Putin said that Russia's Hmeimim air base in Latakia province and its Mediterranean naval base at Tartus would continue to operate as normal. He said both must be protected "from land, air and sea".
Syria's opposition cautiously welcomed the Russian announcement.
"If there is seriousness in implementing the withdrawal, it will give the [peace] talks a positive push," said Salim al-Muslat, spokesman for the opposition umbrella group, the High Negotiations Committee.
The US also gave a guarded response.
"We will have to see exactly what Russia's intentions are," said White House spokesman Josh Earnest.
President Obama has discussed the development with Mr Putin in a phone call, the White House and the Kremlin said, giving few details.
What did Russia achieve in Syria?
- Russian aircraft flew more than 9,000 sorties
- Destroyed 209 oil production and transfer facilities
- Helped Syrian government troops to retake 400 settlements
- Helped Damascus to regain control over more than 10,000 sq km (3,860 sq miles) of territory
Source: Russian Defence Minister Sergei Shoigu, quoted by Russian media
Separately, US officials said Washington had received no advance warning of Mr Putin's statement.
BBC diplomatic correspondent Jonathan Marcus says Russia has achieved its main goals in the intervention - consolidating President Assad's position, enabling his forces to re-take key pieces of strategic territory and ensuring that Mr Assad remains a factor in any future Syrian settlement.
Russia has long insisted its bombing campaign only targets terrorist groups but Western powers have complained the raids hit political opponents of President Assad.
The UN's envoy to Syria, Staffan de Mistura has meanwhile described the latest peace talks as a "moment of truth".
He said there was no "plan B" should the talks fail, with the only alternative a return to war.
A cessation of hostilities agreed by most participants in the conflict began late last month - but there have been reports of some violations on all sides.
In the latest fighting, Syrian government forces are reported to have advanced on so-called Islamic State's (IS) positions near the world heritage site of Palmyra.