Russian forces are preparing to start leaving Syria after Monday's surprise withdrawal announcement by President Vladimir Putin.

Defence Minister Sergey Shoigu gave the order to withdraw and aircraft are being loaded up for long-range flights back to Russia, the ministry said.

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.

Meanwhile, a UN commission will present a report on war crimes in Syria later.

The Russian force reduction was announced during a meeting between 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No details have been given on how many planes and troops would be withdrawn from the Hmeimim base, in Latakia province, or a deadline for completing the pullout.

But the defence ministry said in a statement on its website that the "main part" of the Russian armed forces group in Syria was being relocated to the Russian Federation.

"Technical staff of the airbase started preparing aircraft for the long-range flight to the Russian Federation airfields," it said.

"The personnel are loading equipment, logistics items and inventory into transport aviation aircraft."

Aircraft from the base would make the flight to Russia - more than 5,000km - in formation accompanied by the transport planes with fuel stops at other Russian bases along the way, it added.

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 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.

Grey line

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.

Putin, however, said Hmeimim and Russia's 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.

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).

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.

In a phone call, 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.