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," 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 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 Putin in a phone call, the White House and the Kremlin said, giving few details.

(BBC)