The Western-backed Syrian opposition has threatened to walk out of the Geneva 2 peace talks over a UN invitation to Iran.
The Syrian National Coalition, which had only just agreed to attend the conference, set a deadline for the invitation to Tehran to be withdrawn.
The invitation to Iran, an ally of President Bashar al-Assad's regime, was announced by UN secretary Ban Ki-moon, after Iranian foreign minister Mohammed Javad Zarif promised to play a "positive and constructive role".
"He has assured me that, like all the other countries invited to the opening-day discussions in Montreux, Iran understands that the basis of the talks is the full implementation of the 30 June, 2012, Geneva communique," Ban said.
The move also enraged historic enemy Saudi Arabia, which said that Iran was not eligible to attend.
"Inviting any party to the Geneva 2 talks is dependent on their support of a transitional government," a statement from the Saudi government read. "Iran has not voiced its support for a transitional Syrian government and has troops fighting alongside Assad."
France and United States said that Iran was not welcome unless it publicly supported a transitional government in Syria.
"This is something Iran has never done publicly and something we have long made clear is required," US State Department spokeswoman Jen Psaki said. "If Iran does not fully and publicly accept the Geneva communiqué, the invitation must be rescinded."
Iran said it would attend the talks without preconditions.
"We have always rejected any precondition for attending the Geneva 2 meeting on Syria. Based on the official invitation we have received, Iran will attend the Geneva 2 without any preconditions," Foreign Ministry spokeswoman Marzieh Afkham said.