Russian forces are due to start leaving Syria after Monday’s surprise withdrawal announcement by President Vladimir Putin.
Western officials have cautiously welcomed the move, saying it could put pressure on the Syrian government to engage in negotiations.
Peace talks aimed at resolving the five-year conflict are due to enter a second day in Geneva.
Meanwhile, a UN commission will present a report on war crimes in Syria later.
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No details were given on how many planes and troops would be withdrawn or a deadline for completing the pullout.
Bigger game: BBC’s Lyse Doucet in Damascus
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 Russia’s Hmeimim air base in Latakia province and its Mediterranean naval base at Tartus would continue to operate as normal.
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.
“The whole subject happened in complete co-ordination between the Russian and Syrian sides, and is a step that was carefully and accurately studied for some time,” Damascus said.
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
In Geneva, talks aimed at ending the conflict continue, with the UN envoy for Syria Staffan de Mistura expected to meet the opposition umbrella group, the High Negotiations Committee (HNC).
Mr de Mistura had earlier described the efforts as a “moment of truth”, and warned there was no “plan B” should the talks fail, with the only alternative a return to war.
Most participants in the 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.
‘Too early to say’
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 received a guarded welcome from Western diplomats and the Syrian opposition.
“If there is seriousness in implementing the withdrawal, it will give the [peace] talks a positive push,” said Salim al-Muslat, spokesman for the HNC.
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.
UK Foreign Secretary Philip Hammond said the move could be positive if it marked a real commitment to a political transition in Syria.
The German Foreign Minister, Frank-Walter Steinmeier, said it would increase pressure on Mr Assad to finally and seriously negotiate.