Kerry heads to Moscow as Russian pullout boosts Syria peace

US Secretary of State John Kerry will visit Russia next week to discuss the Syria crisis, Washington has said, after Moscow began withdrawing most of its forces from the country.

Kerry said he would hold talks with Russian President Vladimir Putin to "try to take advantage of this moment," which he described as the best opportunity in years to end the bloodshed.

Russia's withdrawal has spurred hopes for peace talks being held in Geneva to try to end Syria's brutal five-year conflict, which has killed more than 270,000 people and displaced millions.

Putin ordered the "main part" of Russia's forces out of Syria on Monday, but pledged to keep some air and naval bases in the war-torn nation.

A senior official said on Tuesday strikes would also continue against "terrorist targets", and a monitoring group said Russian aircraft had struck the Islamic State group around the ancient city of Palmyra.

UN envoy Staffan de Mistura described the withdrawal as a "significant development" for the talks, after the regime and rebel delegations submitted their roadmaps for a political solution.

"We hope (this) will have a positive impact on the progress of the negotiations," he said.

The White House said "the earliest indications are that the Russians are following through" on the withdrawal, and some Russian aircraft had left Syria.