GENEVA: Syria’s government and opposition figures were expected to respond Wednesday to a “surprise” UN proposal on mapping a way to a new constitution, the second day of renewed peace talks in Geneva.

The sixth round of UN-backed negotiations is the latest drive to bring a political solution to the conflict which has claimed more than 320,000 lives.

It began amid rising tensions over a US charge that Syrian President Bashar Assad’s government was using a prison crematorium to hide evidence of thousands of murdered detainees.

UN envoy Staffan de Mistura met twice on Tuesday with both the Syrian government delegation and the opposition High Negotiations Committee (HNC), with meetings at the UN headquarters running late into the evening.

Opposition members said their meetings focused on the issue of thousands of detainees still held in Syrian jails as well as the drafting of a new constitution.

De Mistura presented the HNC with a document proposing a team of civil society activists and technocrats responsible for mapping a way forward to a draft constitution, two opposition sources told AFP.

The “consultative” team would begin work immediately on “specific options for constitutional drafting,” according to a copy of the proposal seen by AFP.

It would aim to “prevent a constitutional or legal vacuum at any point during the political transition process being negotiated.”

But the HNC’s Munzer Makhos said opposition figures had “many reservations” and were still discussing it.

“It will become clear on Wednesday. This paper was a surprise — it was not expected in the first place,” Makhos said.

Syrian government officials did not respond to requests for comment, but a source close to their delegation confirmed they had received a copy of the document.

The UN-backed talks are expected to focus on four separate “baskets“: governance, a new constitution, elections and combating “terrorism” in the war-ravaged country.

While the HNC has insisted on Assad’s ouster as part of any political transition, the government sees this demand as a nonstarter.

The back-to-back meetings on Tuesday were brief, in line with de Mistura’s pledge to keep the talks “rather business-like, rather short.”

Hopes for a breakthrough remain dim, particularly after US claims of new regime atrocities at the notorious Saydnaya prison near Damascus.

Satellite images appear to show snow melting on the roof of a building and heavy-duty ventilation systems attached to the military complex, which appear to support earlier claims by rights groups that Saydnaya is an execution center.

Damascus was swift to reject the claim, with the foreign ministry calling it “nothing but the product of the imagination of this administration and its agents.”

HNC spokesman Salem Al-Meslet, speaking to AFP ahead of his delegation’s first meeting on Tuesday, said the new accusations demanded a response.

“The Americans know what’s going on in Syria now... To save the lives of Syrian people it needs some action from the (United) States, from our friends, and I hope they will do it very soon,” he said.

The Geneva talks have been overshadowed by a string of rebel evacuations from the Syrian capital and parallel negotiations in the Kazakh capital, Astana.

Sponsored by rebel supporter Turkey and regime backers Russia and Iran, the Astana negotiations produced a May 4 deal to create four “de-escalation” zones across some of Syria’s bloodiest battlegrounds.

De Mistura has dismissed suggestions that the Astana talks are competing with the Geneva track, saying they were “working in tandem.”

The HNC insisted that UN-backed talks are by far the most critical.

“We only believe in deals that are agreed upon here in Geneva — not in Astana,” Meslet said.