The UN is calling for a "firewall" around the Syria process, after diplomatic talks for the conflict-torn country were postponed over Russian demands reportedly linked to its war in Ukraine.

The UN Special Envoy for Syria, Geir Pedersen, had been set to host a ninth round of talks in Geneva towards a new Syrian constitution this week.

But he was forced to postpone after Moscow raised issues over the venue, reportedly insisting that Switzerland, which has imposed sanctions over the Ukraine war, is not neutral.

Spokeswoman Jenifer Fenton told reporters in Geneva that Pedersen had briefed the Security Council behind closed doors Thursday, and had described the postponement as "unfortunate and disheartening, particularly since it results from matters unrelated to the Syria file."

He had told the council that a Syrian government representative had informed him that it would only be ready to participate in the next round of constitutional talks "when what he described as requests made by the Russian Federation have been met," she said.

Pedersen, she said, "urged all parties to firewall the Syrian process from the impact of the conflicts elsewhere in the world and put the interest of Syrians first."

Fenton highlighted that back in 2018 when the constitutional committee was conceived, one of the conditions was that it "would hold its meetings in Geneva without foreign interference".

Pedersen acknowledged that the challenges facing the long-stuck talks aimed at rewriting Syria's constitution "run much deeper than issues of venue," she said.

"One challenge is that of pace and working methods, and another challenge, is that of political will."

During the last round of talks in early June, the rival parties made very little headway, with Pedersen's office decrying the slow pace and acknowledging at the time that "differences remained significant".

On Thursday, Pedersen told the council that the "current deadlock ... calls for a broader reflection," Fenton said.

He had said the situation for Syrians remains "dire", and had stressed the need to "put aside our differences and offer Syrians a common and constructive position."

Syria's war, which erupted in 2011 after the brutal repression of anti-government protests, has killed nearly half a million people and displaced millions.

The tentative negotiations are aimed at rewriting the war-torn country's constitution. It is hoped the talks could pave the way towards a broader political process.

Norwegian diplomat Pedersen mediates at the talks between representatives each from President Bashar al-Assad's government, the opposition, and civil society.