Russian forces have started leaving Syria after Monday's surprise withdrawal announcement by President Vladimir Putin.
The first group of aircraft left Hmeimim air base in Syria on Tuesday morning, the Russian defence ministry said (in Russian).
Western officials cautiously welcomed the move, saying it could pressure Syria's government to engage in talks.
Peace talks aimed at resolving the conflict are entering a second day.
UN Special Envoy Staffan de Mistura, who is mediating in the talks, welcomed the Russian decision.
"The announcement by President Putin on the very day of the beginning of this round of Intra-Syrian Talks in Geneva is a significant development, which we hope will have a positive impact on the progress of the negotiations," he said.
The Russian force reduction was announced during a meeting between Mr Putin and his defence and foreign ministers.
Russia is a key ally of Syrian President Bashar al-Assad, and his office sought to reject speculation there was a rift between the two countries, saying the move was mutually agreed.
The Russian air campaign started last September, tipping the balance in favour of the Syrian government and allowing it to recapture territory from rebels.
No details have been given on how many planes and troops would be withdrawn from the Hmeimim base, in Latakia province, or a deadline for completing the pullout.
"The first group of Russian planes has flown out of the Hmeimim air base for their permanent bases on the territory of the Russian Federation," the defence ministry statement said.
Russian TV earlier showed aircraft being refuelled and crates being loaded with equipment.
Aircraft from the base would make the flight to Russia - more than 5,000km - in small groups each led by Il-76 or Tu-154 transport planes, the ministry added.
They would then go their separate ways to their own bases after crossing the Russian border, it said.
Su-24 tactical bombers, Su-25 attack fighters, Su-34 strike fighters and helicopters were returning home, the TV said.
Russia's military intervention bolstered president Assad's forces on key front lines where they were close to collapse.
Russia now wants to see an end to this war - and it is known to be concerned about the Syrian government's tough line on talks which have just resumed in Geneva, as well as president Assad's recent comments in an interview that he would one day take back, militarily, all the territory he lost.
That is not a war president Mr Putin can afford to be part of.
And he has a bigger game here - his wider relationship with the West and most of all Washington which is also anxious to find a way out of this crisis in Syria - as hard as that is.
It is not clear how many military personnel Russia has deployed, but US estimates suggest the number ranges from 3,000 to 6,000, AP reports.
Mr Putin, however, said Hmeimim and Russia's Mediterranean naval base at Tartus would continue to operate as normal.
Senator Viktor Ozerov, head of the Federation Council defence committee, said as many as two battalions - some 800 servicemen - could remain in Syria after the withdrawal to guard the two bases, Interfax news agency reported.
Military advisers training Syrian government troops would also stay, he added.
Russia had long insisted its bombing campaign only targeted terrorist groups but Western powers had complained the raids hit political opponents of President Assad.
In a statement, the Syrian government said the plan was agreed between the two countries.
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
Most participants in the Syria conflict agreed to a cessation of hostilities, which has been largely holding despite reports of some violations on all sides.
Meanwhile, the UN Commission of Inquiry on Syria will present its report on war crimes committed by all sides in Syria's war to the UN Human Rights Council on Tuesday in Geneva.
In a phone call, Mr Putin and US President Barack Obama discussed the situation in Syria and the "next steps required to fully implement the cessation of hostilities" agreed last month, the White House said.
The Kremlin said both "called for an intensification of the process for a political settlement" to the conflict.
The Russian move has received a guarded welcome from Western diplomats and the Syrian opposition.
An unnamed US official quoted by Reuters said Washington was encouraged by the Russian move, but it was too early to say what it means or what was behind it.