Tillerson discusses Syria with Russia's Lavrov

US Secretary of State Rex Tillerson discussed the conflict in Syria with Russian Foreign Minister Sergey Lavrov on Friday, the State Department said.

Their conversation took place a day after Russia, Iran and Turkey signed a deal setting up four "de-escalation zones" in Syria during talks in Kazakhstan's capital Astana. The United States did not take part.

"The secretary of state spoke by phone with Russian Foreign Minister Lavrov today about the efforts to de-escalate the ongoing conflict in Syria," State Department spokeswoman Heather Nauert said in a statement.

Experts are sceptical about Thursday's Kremlin-brokered deal because neither the Syrian government nor the rebels were direct signatories and the opposition offered only a lukewarm reaction.

Washington gave the deal an extremely cautious welcome, citing concerns about Iran's role as a guarantor even as it expressed hope the agreement could set the stage for a later settlement.

The United States takes part in separate peace talks under a UN mandate in Geneva, where the rivals have been deadlocked on key issues.

A new round of Geneva talks is set for later this month.

"The secretary looks forward to further meetings with the foreign minister to discuss the respective roles of the United States and Russia in de-escalating the conflict and supporting the talks in Geneva to move the political solution forward," Nauert said.

More than 320,000 people have died in Syria since the country's war began with anti-government protests in March 2011.