Long-awaited peace talks on Syria's devastating civil war are set to go ahead this week after the UN withdrew an invitation to Iran, but Tehran says they are doomed to fail.
Preparations were under way in the Swiss city of Montreux for Wednesday's start of the so-called Geneva II talks, after the abrupt United Nations reversal averted a Syrian opposition boycott.
UN leader Ban Ki-moon late on Monday withdrew his surprise invitation to Iran, a major backer of President Bashar al-Assad, less than 24 hours after he announced it.
Iran was quick to respond, with Deputy Foreign Minister Abbas Araqchi telling state television there was little chance of peace without Tehran's involvement.
"It is clear that a comprehensive solution to the Syria issue will not be found when all influential parties are not involved in the process," Araqchi said.
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"Everyone knows that without Iran the chances of a real solution to Syria are not that great."
Iranian Foreign Minister Mohammad Javad Zarif claimed the invitation had been withdrawn "under pressure", saying he had earlier made it clear to Ban that Tehran "does not accept any preconditions" to the talks.
"Iran was not too keen on attending in the first place," Zarif added in comments reported by the ISNA news agency.
The US, which is organising the talks along with Russia and the UN, had said Iran could not attend unless it agreed to the principle of creating a transitional government set out in June 2012 peace talks in Geneva.
Moscow, another key Assad ally, had pushed for Tehran to take part and Foreign Minister Sergei Lavrov said on Tuesday it had been an error to withdraw the invitation.
"Of course this is a mistake," Lavrov told reporters.
But he also downplayed the move, saying "no disaster has happened" and describing the peace talks as "largely ceremonial".
This week's talks will be the most intensive diplomatic effort yet to resolve Syria's civil war, which after nearly three years has left more than 100,000 dead and millions forced from their homes.
US Secretary of State John Kerry departed Washington before dawn on Tuesday to head to Switzerland, with officials warning the talks were just the start of a long and grinding process.
In Montreux, police cordons had been set up outside the conference venue on Lake Geneva and security was tight, with helicopters flying overhead.
