The main Syrian opposition group has arrived in Geneva, a day after backing down from their threat to boycott the UN-sponsored peace talks.
But a spokesman said they stood by their demand for an end to air strikes and blockades before they will negotiate with the Syrian government.
Their delegation is due to meet UN envoy Staffan de Mistura on Sunday.
More than 250,000 people have died and 11 million have fled their homes in almost five years of war in Syria.
The Syrian civil war has also been the biggest driver behind Europe's migration crisis.
On Saturday, at least 39 migrants, among them children, drowned in the Aegean Sea while trying to cross from Turkey to the Greek island of Lesbos.
The Saudi-backed opposition High Negotiations Committee (HNC) finally agreed late on Friday night that they would come to the talks in Geneva - hours after the Syrian government delegation had already arrived and held preliminary talks with Mr Mistura.
Geneva peace talks
Who is attending?
Delegates from the Syrian government and the main opposition bloc, the High Negotiations Committee (HNC). Other opposition figures have been invited as advisers. Who is not attending? Syrian Kurdish group the PYD (regarded by Turkey as a terror group); so-called Islamic State (IS); al-Nusra Front.
What is being discussed?
A possible ceasefire; release of prisoners; aid deliveries to the worst-hit areas; the threat from IS.
Upon his arrival in Geneva, HNC spokesman Salim Muslet said they wanted the release of women and children from government jails, the end to air strikes and aid to be allowed into besieged towns - but that they were not a precondition of talks.
"We're always optimistic but the problem is we're facing a dictatorship there in Syria. Really, if he's willing to solve these problems, we wouldn't have seen these crimes in Syria, these massacres," he told reporters.
But Riad Hijab, the head of the HNC who is not in Geneva, warned in an online post that the delegation's presence "will not be justified" if the Syrian government continues to "commit these crimes".
