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 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.
"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Iranian Foreign Minister Mohammad Javad Zarif claimed the invitation had been withdrawn "under pressure", saying he had earlier made it clear to Ban that Tehran "does not accept any preconditions" to the talks.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 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 Tehran to take part and Foreign Minister Sergei Lavrov said 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" 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 100,000 dead and millions forced from their homes.
Months of diplomatic wrangling were needed to convince all sides to take part, with the main opposition group, the Syrian National Coalition, only agreeing at the last minute.
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.
Need 'for patience and for persistence'
"Everybody has to understand that this is the beginning of a process. It's not going to be fast. It's very bitter fighting on the ground.
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"And so there's going to be an absolute requirement for patience and for persistence."
Deep divisions within the opposition were exposed ahead of the talks and questions have been raised about whether its delegation will be truly representative of Assad's opponents.
The biggest bloc in Syria's opposition-in-exile, the Syrian National Council, said on Monday that it was quitting the coalition because taking part in the talks would renege on its "commitments" not to enter negotiations until Assad left power.
In an AFP interview published on Monday, Assad bluntly ruled out a power-sharing deal.
He insisted the peace conference should focus on what he called his "war against terrorism".
Assad dismissed the opposition as having been created by foreign backers and said nothing could stop him from seeking a new term as president in an election he wants to hold in June.
After the launch of the conference in Montreux, which will also bring together some 40 countries and regional bodies, the Syrian government and opposition are to start face-to-face talks in Geneva on Friday.
In Montreux, police cordons had been set up outside the conference venue on Lake Geneva and security was tight, with helicopters flying overhead.
Lavrov and Kerry were to meet in Montreux late Tuesday for pre-conference talks.
Security officials said the arrangements had been complicated by demands from participants.
"There are some delegations that want to be physically separated -- who cannot be in the same hotel, who do not want to be in the same hotel," local police commissioner Jean Christophe Sauterel told AFP.
Adding to the urgency of the talks, three former international prosecutors have accused the Assad government of large-scale killing and torture.
Their report — revealed Monday by the Guardian newspaper and CNN television and based on the evidence of a military police defector — said about 55,000 digital images of 11,000 dead detainees had been handed over.
One of the report's authors — Desmond de Silva, the former chief prosecutor of the special court for Sierra Leone — told the Guardian that the evidence showed that Assad's forces had carried out "industrial-scale killing".