The long-awaited talks, which open on the eve of the fifth anniversary of the outbreak of the conflict, mark the latest bid to end the bloodshed that has killed more than 2,70,000 people and displaced millions.
Hours before negotiations were due to start, what would be discussed was still unclear, and Western powers hit out at the regime for saying that removing President Bashar al-Assad would be a "red line".
He urged the regime's key ally Russia to bring Damascus into line, saying Assad was trying to "to take off the table something that (Russian President Vladimir) Putin and Iran had committed to".
"This is a moment of truth, a moment where all of us have to be responsible," he said.
French Foreign Minister Jean-Marc Ayrault went further, calling Syrian Foreign Minister Walid Muallem's comments a "provocation" and a "bad sign" in the peace efforts for Syria.
While analysts say much has changed since the last round of indirect talks collapsed in February, Assad's fate and whether elections will be held within 18 months remain huge obstacles to agreeing a roadmap to peace.
A temporary ceasefire introduced on February 27 has largely held, despite accusations of violations from both sides, offering some reprieve for Syria's war-ravaged people and allowing aid to reach some 150,000 living under siege.