The Geneva peace conference scheduled for January 22 is aimed at ending the nearly three-year-old civil war, a bloody stalemate which has killed an estimated 120,000 people and driven millions from their homes.
But the opposition National Coalition, an umbrella group increasingly at odds with rebels on the ground, has insisted Assad have no role in the country's future, a demand long rejected by Damascus, casting doubt on whether a middle ground can be found.
Iran - a key ally of the Syrian regime which penned a landmark nuclear agreement with world powers on Sunday - said today it was ready to take part in the so-called Geneva II conference, but would not accept any preconditions.
A day after the United Nations announced the date of the talks, the Coalition affirmed its "absolute rejection of Assad or any of the criminals responsible for killing the Syrian people playing any role in a transitional body... Or in Syria's political future."
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The opposition also called on world powers to "ensure humanitarian supplies reach all areas of Syria, while all prisoners must be set free" and that there should be "an immediate end" to massacres.
Rebel chief Selim Idriss said the Free Syrian Army would be ready to go to the talks if "the demands of revolutionaries on the ground," including Assad's fall, are met.
But while both the coalition and the FSA enjoy Western support, it's unclear how much control they have over the hundreds of rebel groups fighting on the ground, which include increasingly powerful jihadists battling both the regime and other rebel groups.