Staffan de Mistura. Photo: AFP

UNITED NATIONS: The UN?s mediator on Syria asked the Security Council on Tuesday to help break the deadlock in peace talks by providing some ideas on drafting a new constitution and elections, key elements of a future settlement.

Staffan de Mistura told the council that a "golden opportunity" to make progress toward a peace deal had been missed during the latest round of talks that ended in Geneva last week.

"I believe the time has come for the UN to provide some specific elaborations on the constitutional and electoral baskets," the envoy said.

"Since we have been hearing and talking about constitutional process and election and we were not able in the intra-Syrian talks to actually engage both sides on it, well let me get you on it," he told the council.

The envoy was to meet UN Secretary-General Antonio Guterres later Tuesday to discuss the failure of the talks in Geneva as Russia, Iran and Turkey gear up for a fresh round of meetings in the Kazakh city of Astana this week.

De Mistura said the proposals on the constitution and elections would "stimulate a wider consultation" on the way forward.

Peace talks have repeatedly stumbled over the fate of President Bashar al-Assad, with negotiators from Damascus refusing to meet the opposition directly until it drops demands that he leave office.

Russian President Vladimir Putin has stepped up his Syria diplomacy in recent months, setting up four "de-escalation" zones inside Syria and proposing to hold a peace conference.

UN diplomats are increasingly concerned that Russia is seeking to set up an alternate track to the Geneva talks to reach a settlement that would suit its ally Assad.

Assad?s forces have regained the upper hand in the war, backed by Russia?s military intervention, retaking large swatches of rebel-held territory.

More than 340,000 people have died in Syria?s nearly seven-year war and over half of the population has been driven from their homes.