Syrian President Bashar al-Assad said Thursday he has never faced pressure from Russia to step aside, as Moscow prepared to host US Secretary of State John Kerry on reviving peace efforts.

Speaking to NBC News in Damascus, Assad insisted his Russian counterpart Vladimir Putin and Foreign Minister Sergei Lavrov had never raised the issue of his departure or a political transition.

"Only the Syrian people define who's going to be the president, when to come, and when to go. They never said a single word regarding this," he said.

Assad's fate is a key question in efforts to bring about a negotiated settlement to Syria's five-year civil war.

Moscow and Washington have backed a roadmap that calls for a nationwide ceasefire and Geneva-based talks on a "political transition".

But there has been little progress towards a hoped-for resumption of talks this month, and the prospects for such a transition now appear slim.

UN Syria envoy Staffan de Mistura on Thursday urged Moscow and Washington to push for a resumption of the talks next month.

The talks "have a target date of August," De Mistura said, adding that they need to be "a credible beginning of a roadmap towards a political transition."

Kerry was due to arrive later Thursday in Moscow, a close ally of Assad's government that launched air strikes in support of regime forces in September last year.

(AFP)