MONTREUX, Switzerland (AFP) - Long-awaited peace talks on Syria's devastating civil war are set to go ahead this week after the United Nations withdrew an invitation to Iran, but Teheran said on Tuesday they were doomed to fail.

Officials have expressed little hope of a breakthrough at the peace conference starting on Wednesday in Switzerland, but have said just bringing President Bashar al-Assad's regime and the opposition to the same table will be a sign of progress.

There were stark reminders of the war's impact in the run-up to the talks, with a bombing in Beirut that left four dead and new evidence alleging that Assad's regime has systematically killed and tortured thousands.

Preparations were under way in the Swiss town of Montreux for the start of the so-called Geneva II talks after the abrupt United Nations reversal on Iran averted a Syrian opposition boycott.

UN leader Ban Ki Moon late on Monday withdrew a surprise invitation to Iran, a major backer of Mr 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 Teheran'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," Mr Araqchi said.

"Everyone knows that without Iran the chances of a real solution to Syria are not that great."

The United States, 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 Teheran to take part and Foreign Minister Sergei Lavrov said it had been an error to withdraw the invitation.

"Of course this is a mistake," Mr Lavrov told reporters.

But he also downplayed the move, saying "no disaster has happened" and describing the peace talks as "largely ceremonial" and a "one-day event".

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 130,000 dead and millions forced from their homes.

US Secretary of State John Kerry departed Washington before dawn Tuesday to head to Switzerland, with officials warning the talks were just the start of a long and grinding process.

"I don't think anyone who's dealt with Syrian officials has any false expectations of rapid progress," a senior State Department official said.