UN Special Envoy for Syria Staffan de Mistura on Tuesday met a Syrian regime delegation at the first day of a sixth round of peace talks in Geneva.

The new round of talks is expected to end on Friday. The delegation from the Syrian regime is headed by Bashar al-Ja'aafari. The opposition is led by Nasr al-Hariri from the High Negotiations Committee.

De Mistura will meet separately with each delegation at UN offices.

The UN negotiations are focused on four separate "baskets": governance, a new constitution, elections and combating "terrorism" in the war-ravaged country.

With Assad's negotiators and the HNC expected to be in the Swiss city until the weekend, de Mistura said he wanted to drill down on several issues in hopes of generating solid proposals.

But one issue -- Assad's fate -- remains a daunting roadblock. The HNC has insisted the president's ouster must be part of any political transition, a demand unacceptable to the Syrian regime.

Upon arrival in Geneva Hariri said: "The road to Syria's freedom runs through Geneva. That is why we have come here, committed to negotiate a political solution. And that is why [Bashar al-] Assad is frightened of this process. He is afraid to engage with the agenda of political transition."

"We will engage constructively [with] the UN Special Envoy's agenda. The key to progress is political transition, to a free Syria that has no place for terrorism and Bashar al-Assad," Hariri added.

Assad has recently called the Geneva process "null", telling Belarus's ONT channel that it had become "merely a meeting for the media."

Assad has however given more credit to a separate diplomatic track in Kazakhstan's capital Astana, which is being led by his allies Russia and Iran along with opposition supporter Turkey.

The Astana track produced a May 4 deal to create four "de-escalation" zones across some of Syria's bloodiest battlegrounds.

The UN's Syria envoy Staffan de Mistura has dismissed suggestions that the Astana negotiations were overshadowing the Geneva track. "We're working in tandem" he told reporters on Monday.

Five previous rounds of UN-backed negotiations have failed to yield concrete results and hopes for a major breakthrough remain dim. The last round of talks began on March 23 and ended on March 31 without any clear result.

Since civil war in Syria erupted in March 2011, more than 250,000 people have been killed, according to the UN. The Syrian Center for Policy Research puts the death toll at more than 470,000.

Despite the heavy toll of the war on both sides, the momentum is also on the regime's side. Over the last year, Assad has strengthened his position on the ground, with the opposition reeling from a major setback in the capital Damascus.

The opposition position has weakened since the last round ended on March 31 after the government secured the evacuation of three opposition-held districts, bringing it closer to exerting full control over the capital for the first time since 2012.

Another shifting force influencing the talks is the role of the United States, an erstwhile opposition supporter that largely withdrew from the process under President Donald Trump.

De Mistura said Monday he was "encouraged by the increasing engagement, the increasing interest, by the US administration in finding a de-escalation".

However, Washington late Monday warned Russia to not turn a blind eye to Assad's alleged crimes, with the State Department releasing satellite images that it said backed up reports of mass killings at a Syrian jail.

The head of the opposition delegation to the talks welcomed the US statement, but complained it had come too late.

"This is but a drop in the ocean. What happens in the regime's prisons is much uglier than this," Nasr al-Hariri said.