At the heart of the dispute is whether Iran has accepted the terms of the talks, which begin Wednesday in Montreux, Switzerland: to establish “by mutual consent” a transitional body to govern Syria. Mr. Ban said he had been privately assured that Iranian officials “welcome” those rules and that they had pledged to play “a positive and constructive role.”
American officials said they had been in regular communication with the United Nations over the requirements Iran would need to meet to be invited, but they appeared to have been caught off guard by Mr. Ban’s hastily organized news conference. They pointed out that Iran had not publicly accepted the formal mandate for the conference, which was agreed upon in Geneva in 2012 and is known as the Geneva communiqué.
“If Iran does not fully and publicly accept the Geneva communiqué, the invitation must be rescinded,” Jen Psaki, a State Department spokeswoman, said in a statement.
Officials in Washington emphasized that Iran had made no such public statement at the time of Mr. Ban’s news conference. It was expected to release one early Monday.
If Iran has accepted the Geneva terms, it would be a sharp turnaround, since it has long insisted that it will participate in talks only if there are no preconditions. Still, such a shift would not necessarily mean Tehran had accepted that President Bashar al-Assad must leave office.
Some 30 countries have been invited to Montreux for what may be a largely ceremonial opening day of the peace talks. Two days later, Syria’s government and opposition delegations will move to Geneva to continue the deliberations, mediated by a United Nations special envoy, Lakhdar Brahimi.
The United States has long been wary of Iran’s intentions. Tehran has been one of the Assad government’s staunchest political and military supporters, sending arms to Damascus and encouraging Hezbollah, the Lebanese Shiite militia, to join the fight on the side of Mr. Assad.
As recently as last Monday, Secretary of State John Kerry complained that Iran was, effectively, a belligerent in the conflict.
“Iran is currently a major actor with respect to adverse consequences in Syria,” Mr. Kerry said. “No other nation has its people on the ground fighting in the way that they are.”
On Sunday, Ms. Psaki added in her statement, “We also remain deeply concerned about Iran’s contributions to the Assad regime’s brutal campaign against its own people, which has contributed to the growth of extremism and instability in the region.”
“Given that Iranian forces and their Shia militias are deployed on the ground backing up Assad, it means another Assad backer will be present at this meeting,” Mr. Tabler said.
Syria’s political opposition said in a Twitter message that it would not attend unless Mr. Ban withdrew Iran’s invitation.
“The Syrian coalition announces that they will withdraw their attendance in Geneva 2 unless Ban Ki-moon retracts Iran’s invitation,” the Twitter message said, quoting Louay Safi, a coalition spokesman.
The ultimatum came just a day after the coalition, facing a boycott by a third of its members, had voted to send a delegation to the peace talks. The opposition has been under intense international pressure, including from the United States government, to participate.
Mr. Ban said Sunday that he had spoken extensively with Iran’s foreign minister, Mohammad Javad Zarif.
“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 communiqué,” Mr. Ban said.
“Foreign Minister Zarif and I agreed that the goal of the negotiations is to establish by mutual consent a transitional governing body with full executive powers,” he added. “It was on that basis that Foreign Minister Zarif pledged that Iran would play a positive and constructive role in Montreux.”