"We do have a principled position in the United Nations that no amnesties should be considered for those suspected of having committed crimes against humanity or war crimes," UN rights chief Zeid Ra'ad al-Hussein told reporters
The Geneva talks represent the biggest push to date to chart a way out of the tangled almost five-year-old Syrian war that has killed more than 260,000 people and forced millions from their homes.
The urgency to find a solution was brought home Sunday when multiple explosions killed 71 people near a revered Shiite shrine outside the capital Damascus, according to a Britain-based monitor.
The blasts were claimed by Islamic State, the extremist group that is running swathes of Syria and Iraq and which has launched attacks worldwide, including in Paris in November when 130 people were murdered.
UN envoy Staffan de Mistura's aim is to coax the warring sides into six months of indirect "proximity talks" as part of an ambitious roadmap agreed by the many outside powers embroiled in the war.
The plan, hammered out in Vienna in November, envisions elections within 18 months, but the key question of the future of President Bashar al-Assad — backed by Iran and Russia — was left until later to resolve.
Representatives for Assad arrived in Geneva on Friday followed reluctantly a day later by the opposition umbrella group the High Negotiations Committee (HNC).
But the Saudi-backed HNC is refusing to play ball unless Assad's regime first allows humanitarian access to besieged towns, stops bombarding civilians and releases prisoners.
"We are here for political negotiations but we cannot start those until we have those gestures," HNC spokeswoman Basma Kodmani told reporters on Sunday.
A Western diplomat said that on Sunday de Mistura made proposals to what he called a mistrustful HNC in talks in a Geneva hotel, but that they were still hesitating.
"We only came to Geneva after written commitments on the fact that there would be serious progress on the humanitarian issues," Kodmani said.
Damascus's chief negotiator Bashar al-Jaafari —who met de Mistura on Friday — on Sunday called the opposition "not serious".
"We do not know who is the other side," the tall and stern Syrian ambassador to the UN told a packed news conference.
"They don't even have a final list."
Another meeting tentatively scheduled between de Mistura and Jaafari for Monday morning was postponed until the seasoned Italian-Swedish diplomat meets with the HNC at the UN at 5pm, his office said.
Outside powers were also in Geneva keeping a close eye on proceedings, with Jaafari reportedly meeting with the Russian ambassador and Western envoys touching base with the opposition on Sunday.
A US official said that Anne Patterson, assistant secretary for Near Eastern Affairs and US Syria envoy Michael Ratney were due to meet Russian Deputy Foreign Minister Gennady Gatilov on Monday in Geneva.
US Secretary of State John Kerry called on Sunday both sides "to seize this opportunity", urging the opposition to drop preconditions but also accusing the regime of deliberately starving besieged towns.
"Without negotiations, the bloodshed will drag on until the last city is reduced to rubble and virtually every home, every form of infrastructure, and every semblance of civilization is destroyed," Kerry said.
Another thorny issue in Geneva is which rebel groups will be involved in the talks, although all sides agree on the exclusion of IS and Al-Nusra Front, Al-Qaeda's Syrian branch.
The powerful Army of Islam rebel group is part of HNC's delegation, and the HNC's chief negotiator and Army of Islam member Mohammed Alloush — seen as a "terrorist" by Moscow — was expected on Monday.
In addition, and in a potentially hopeful sign, Western diplomats said Sunday that Riad Hijab, the head of the HNC and a former Syrian prime minister, was also due in Geneva soon.