A new round of Syria peace talks has opened in Geneva as the government in Damascus fiercely denied it used a prison crematorium to hide evidence of thousands of murdered detainees.

Five previous rounds of UN-backed negotiations have failed to yield a solution to the six-year conflict and hopes for a mjor breakthrough remain dim.

Staffan de Mistura, the UN special envoy, met Bashar al-Jaafari, the Syrian government negotiar, on Tuesday, followed by the opposition's High Negotiations Committee (HNC).

Jaafari also met Gennady Gatilov, Russia's deputy foreign minister, before returning to the UN in the evening for further talks, according to Syria's state news agency SANA.

The HNC was also due back to the UN offices in Geneva later on in the evening.

'New Hollywood plot'

Tensions - already high over recent opposition setbacks in Damscus - have been raised further by US claims of new government atrocities at the notorious Saydnaya prison near Damascus.

The US state department on Monday accused Bashar al-Assad's government of using a crematorium to cover up the deaths of thousands of prisoners at Saydnaya.

Syria's foreign ministery on Tuesday "categorically" denied the US accusations , calling them part of "a new Hollywood plot" to justify US intervention in the war-torn country.

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Salem al-Muslet, HNC spokesperson, speaking to AFP news agency before his delegation's first meeting, said the fresh 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.

Assad recently called the Geneva process "null", telling a Belarusian TV network that it had become "merely a meeting for the media".

His comments, coupled with uncertainty over the US' position on Syria, have made "diplomats highly sceptical over the [impact of the talks] in Geneva", said Al Jazeera's Mohammed Jamjoom, reporting from Geneva.

"In fact, it is the most amount of scepticism that I have heard from diplomats involved in this process."

Separate 'baskets'

The negotiations are the latest effort to end a war that has killed nearly half a million people and displaced half of the country's pre-war population.

They are expected to focus on four separate "baskets": governance, a new constitution, elections and combating "terrorism" in the war-ravaged country.

But one issue - Assad's fate - remains a persistent roadblock.

The HNC has insisted Assad's departure must be part of any political transition, a demand unacceptable to the Syrian government.

The HNC's first session with de Mistura focused on releasing detainees and a new constitution, Muslet announced after the meeting.

But the Geneva talks have been overshadowed by a series of rebel evacuations from Damascus and rival negotiations in Kazakhstan's capital, Astana.

Sponsored by Turkey, the opposition's key backer, and Russia and Iran, allies of Assad, that track 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 negotiations were 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," Muslet said.

De Mistura, who has lasted as Syria envoy far longer than his two predecessors, has consistently tried to resist pessimism.

The alternative to talks is "no discussion [and] no hope", he said on Tuesday.

Al Jazeera's Jamjoom said "if de Mistura is not able to come away with something significant from this round of talks, then the whole Geneva process could collapse all together".