In Geneva, UN special envoy Staffan de Mistura held separate talks with representatives of President Bashar al-Assad and with the High Negotiations Committee (HNC) opposition umbrella group.
He said he was "optimistic and determined", but comments from both sides laid bare the enormity of the challenges ahead for ending a highly complex and bitter conflict.
The HNC delegation reluctantly arrived late on Saturday in Geneva and immediately threatened to turn around and leave again unless Assad's "crimes" stop.
Among its demands are that humanitarian aid be allowed to reach hundreds of thousands of people stuck in besieged towns, a halt to the bombing of civilians and the release of hundreds of prisoners.
"We only came to Geneva after written commitments on the fact that there would be serious progress on the humanitarian issues," HNC spokeswoman Basma Kodmani told reporters.
"We are here for political negotiations but we cannot start those until we have those gestures."
- 'Not serious' -
But Damascus's chief negotiator Bashar al-Jaafari denounced the opposition as "not serious".
"We do not know who is the other side. They don't even have a final list," he told a packed news conference.
US Secretary of State John Kerry urged both sides "to make the most of this moment" in an online statement broadcast from Washington.
Kerry urged the opposition to drop their preconditions, but also had hard words for their foes, accusing Assad's forces of deliberately starving besieged towns.
He described the talks in Geneva as being at a "pivotal phase" and said the HNC represented an "inclusive opposition".
De Mistura's office said Sunday that further separate talks would be held on Monday with the opposition and government delegations.
HNC spokesman Munzer Makhous told AFP that the group would stay in Geneva for at least three or four more days.
- Five years -
The Syrian conflict, which began in 2011, has sucked in a wide range of actors from Turkey, Iran and the Gulf states to Western nations and since September, Russia.
The chaos has allowed IS militants to overrun swathes of the country and also in neighbouring Iraq, giving it a launchpad for a series of deadly attacks across the globe including the Paris massacres on November 13 which left 130 dead.
Half of Syria's population have fled their homes, forcing millions to seek refuge in neighbouring countries and also in Europe, where the influx has sparked a major political and social crisis.
The Syrian peace talks, meant to last six months, are part of an ambitious roadmap agreed by all the external powers embroiled in the conflict in Vienna in November, a day after the Paris attacks.
This "Vienna Process" envisages negotiations followed by a transitional government, a new constitution and elections within 18 months, but leaves the question of Assad's future unresolved.
Another thorny issue is which rebel groups will be involved in the talks, although all sides agree on the exclusion of extremists from IS and Al-Nusra Front, Al-Qaeda's Syrian branch.
The powerful Army of Islam rebel group is part of HNC's delegation to Geneva, and the HNC's chief negotiator and Army of Islam member Mohammed Alloush told AFP Sunday that he would be in Geneva the next day.
The choice of Alloush has been controversial, with Syria's ally Moscow saying it considers Army of Islam as "terrorists" and other opposition groups insisting it was unacceptable for the delegation to be led by a member of an armed group.
But Alloush dismissed the criticism on Sunday. "Bashar al-Assad is the one who is the terrorist," he said.