Russia has begun withdrawing some of its troops from Syria, Defence Minister Sergei Shoigu said on Monday.
President Vladimir Putin ordered the partial withdrawal during an unannounced visit to Syria on Monday.
Russian support has been crucial in turning the tide of Syria's civil war in favour of government forces, led by president Bashar al-Assad.
Mr Putin made a similar withdrawal announcement last year, but Russian military operations continued.
When asked how long it would take for Russia to withdraw its military contingent, Mr Shoigu said that this would "depend on the situation" in Syria.
The Russian president was met by Mr al-Assad at the Russian Hmeimim airbase near Latakia.
Mr Putin said: "I order the defence minister and the chief of the general staff to start withdrawing the Russian group of troops to their permanent bases," according to the Russian RIA Novosti news agency.
"I have taken a decision: a significant part of the Russian troop contingent located in Syria is returning home to Russia," he added.
Syria campaign boosts Russian influence
By Steve Rosenberg, BBC Moscow correspondent
Less than a week after announcing he will stand for re-election, Vladimir Putin flies to Syria and declares victory. Coincidence? Probably not.
Signalling the end of Russia's military operation in Syria will go down well with Russian voters.
Electoral concerns apart, Moscow views its two-year campaign in Syria as a success - and not only in terms of fighting international terrorism.
The Russians have succeeded in keeping a key ally, President Assad, in power. In the process, Russia has been guaranteed a long-term military presence in Syria, with its two bases Hmeimim and Tartus. Moscow has also raised its profile across the Middle East.
Then there's the global stage. The operation in Syria prevented Moscow's international isolation.
Russia's annexation of Crimea in 2014 had sparked Western sanctions and earned the country, in the eyes of some Western governments, the label "pariah state". The Syria operation forced Western leaders to sit down and negotiate with Russia's leadership.
Mr Putin said that if "terrorists raise their heads again", Russia would "carry out such strikes on them which they have never seen".
"We will never forget the victims and losses suffered in the fight against terror both here in Syria and also in Russia," he said.
He told President Assad that Russia wanted to work with Iran, the government's other key ally, and Turkey, which backs the opposition, to help bring peace to Syria.
Last week, Mr Putin announced the "total rout" of jihadist militants from so-called Islamic State (IS) along the Euphrates river valley in eastern Syria.
Russia launched an air campaign in Syria in September 2015 with the aim of "stabilising" Mr Assad's government after a series of defeats.
Officials in Moscow stressed that it would target only "terrorists", but activists said its strikes mainly hit mainstream rebel fighters and civilians.
The campaign has allowed pro-government forces to break the deadlock on several key battlefronts, most notably in Aleppo.
The Syrian and Russian air forces carried out daily air strikes on the rebel-held east of the city before it fell in December 2016, killing hundreds of people and destroying hospitals, schools and markets, according to UN human rights investigators.
Moscow has consistently denied that its air strikes have caused any civilian deaths.