The UN-brokered indirect negotiations are due to start tomorrow in Geneva, the latest international push to try to end a war that has killed more than 270,000 people and forced millions from their homes.
After talks with European allies in Paris, US Secretary of State John Kerry hit out at comments by his Syrian counterpart that removing President Bashar al-Assad would cross a "red line" in the negotiations.
Kerry warned Syria and its allies Russia and Iran against "testing boundaries" or lessening their compliance with a fragile February 27 truce brokered by Washington and Moscow that has largely held despite each side accusing the other of violations.
While analysts say much has changed since the last round of talks collapsed in February, the fate of Assad and the holding of elections with 18 months remain huge obstacles to peace.
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Muallem said in Damascus yesterday: "We will not talk with anyone who wants to discuss the presidency... Bashar al-Assad is a red line."
"But the fact is (Assad's) strongest sponsors Russia and Iran have both adopted... An approach which dictates that there must be a political transition and that we must have a presidential election at some time," he added.
Kerry urged Russian President Vladimir Putin to bring the Syrian regime into line, saying he should be concerned that Assad had used his foreign minister "to try and act as a spoiler, to take off the table something that president Putin and Iran had committed to".
Kerry hailed the fact that the ceasefire had led to a reduction of violence of up to 90 per cent, and made possible the delivery of emergency supplies to some 150,000 civilians in besieged areas.
"Despite this progress, we all of us here remain deeply concerned by the Assad regime's practice of removing badly needed medical supplies from those supplies and in particular the surgical kits," he said.