A new round of Syria peace talks opened on Tuesday in Geneva, the latest United Nations push to resolve a six-year conflict that has killed more than 320,000 people.
Five previous rounds of UN-backed negotiations have failed to yield concrete results and hopes for a major breakthrough remain dim.
Syrian President Bashar al-Assad has strengthened his position on the ground, with the rebels reeling from a major setback in the capital Damascus. Assad has also recently called the Geneva process "null", telling Belarus's ONT channel that it had become "merely a meeting for the media".
The Syrian leader 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.
Syrian regime delegation chief Bashar al-Jaafari was holding an initial meeting with de Mistura at the UN on Tuesday.
The main opposition High Negotiations Committee (HNC), led by Nasr al-Hariri and Mohammad Sabra, was due to meet the UN envoy later in the day.
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.
Aron Lund, a fellow at The Century Foundation, said the Geneva talks were revolving around the "dead end" issue of Assad and were not "moving forward in any visible way."
De Mistura, who has lasted as Syria envoy far longer than his two predecessors, has consistently tried to resist pessimism.
The alternative to peace talks is "no discussion (and) no hope", he said.
The opposition position has weakened since the last round ended on March 31 after the government secured the evacuation of three rebel-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.