The United States and United Nations were scrambling Monday to rescue the much-heralded Syrian peace talks which were thrown into disarray by a surprise UN invite to Iran.
The deeply divided Syrian opposition, which had struggled to agree to join the so-called Geneva II conference, is now threatening a boycott unless the invitation to Tehran is withdrawn.
Adding to the tensions, Syrian President Bashar al-Assad bluntly ruled out a power-sharing deal and said in an exclusive AFP interview that the talks opening Wednesday should focus on what he called his "war against terrorism".
UN leader Ban Ki-moon, who sparked the furore with his invitation to Tehran Sunday, said "intensive and urgent" talks were under way with the United States to try to salvage the situation.
But a senior US State Department official insisted: "We expect the invitation will be rescinded."
The Geneva II conference has been nine months in the making and is the most intensive diplomatic effort yet to end the brutal three-year conflict.
And a US official said Washington had received messages from members of the Damascus regime keen to find a peaceful solution to the conflict.
But the opposition National Coalition, holding crisis talks in Istanbul, threw down an ultimatum, saying unless it received confirmation by 1900 GMT that Iran was not invited to Switzerland, "we will not attend."
Washington, London and Paris have said Tehran would have to clearly and publicly support the idea of a transitional government if it was to join the talks.
The surprise invitation came as an interim deal with world powers aimed at containing Tehran's nuclear ambitions came into effect on Monday although it was not clear if the two events were linked.
Iran remained defiant, with a foreign ministry spokeswman saying it will participate "without any preconditions".
Western powers have opposed Iran's presence on the grounds that it had not accepted a communique adopted by major powers in Geneva in June 2012 calling for the creation of an interim government.
Regional powerhouse Saudi Arabia, which supports the Syrian rebels, also said Iran was "unqualified to attend".
The conference is aimed at setting up a transitional government to find a way out of the civil war that has claimed 130,000 lives and created a humanitarian disaster with million of people homeless.
'Car bombing kills five'
But the situation on the ground has become increasingly complicated, with various jihadist groups fighting both against the Assad regime and against more mainstream Islamist factions.
A double suicide car bombing at a border post controlled by moderate Islamist rebels between Syria and Turkey killed at least five people on Monday, a monitoring group said.
The so-called Geneva I accord in 2012 made no mention of Assad's departure, something the Syrian opposition says is non-negotiable.
But Russia, a key sponsor of the talks but also Damascus's main ally, said Tehran's absence would be a "unforgivable mistake".
Ban said after two days of talks with Iranian Foreign Minister Mohammad Javad Zarif that Tehran vowed to play a "very positive and constructive role" and had agreed that the goal of the talks was a transitional governing body with full executive powers.
"I believe strongly that Iran needs to be part of the solution to the Syrian crisis," he said.
But his assurances failed to pacify Western powers concerned over Iran's deployment of military personnel in Syria and its support for Lebanon's Hezbollah movement, which has sent fighters to back up Assad's troops.
The opposition Coalition, which had voted on Saturday to go to Geneva, said in a statement it wanted a clear public commitment from Iran to withdraw all troops and militias and to commit to the terms of Geneva I and II.
"In case of failure to obtain the pledge, we ask (Ban) to rescind/withdraw its invitation to Iran. Otherwise, the Syrian Coalition will not be able to attend the Geneva II conference."
'Regime elements want a way out'
A Western diplomat described the invitation to Iran as a "catastrophic" decision.
In Washington, a US official said he hoped the Swiss talks would provide "an impetus" out of the bloodshed.
"There are elements inside the regime itself, among its supporters, that are anxious to find a peaceful solution, and we've gotten plenty of messages from people inside, they want a way out," the State Department official told reporters.
On Friday, Damascus had offered a number of concessions including a ceasefire plan for Aleppo, a prisoner exchange and steps to improve humanitarian aid.
The moves, announced in Moscow, were seen as another potential diplomatic coup for Russia after it succeeded last year in persuading Damascus to give up its chemical weapons arsenal and avert imminent US strikes.
Both France and Britain said Iran's participation was conditional on clearly accepting the establishment of a transitional government.
And the EU said it hoped Geneva II would be a "first step" to genuine political transition.
But Assad adopted a combative tone in his AFP interview, dismissing the opposition as having been "created" by foreign backers and saying he saw no reason not to seek another term in June's presidential election.
The embattled leader warned of "chaos throughout the Middle East" if his government lost the war.
And he mocked the opposition leaders, saying the notion they could participate in government was "a good joke."