The United Nations' inclusion of Iran in a peace conference aimed at ending the civil war in Syria is threatening to upend the talks before they begin.

The United States is arguing that the invitation must be rescinded because Tehran has refused to agree to the conditions for attending and the main Syrian opposition group has announced it will withdraw if Iran is allowed to participate.

Ban Ki-moon, the United Nations secretary general, over the weekend invited Iran to the Syria peace talks set to begin in Montreux, Switzerland, Wednesday.

Even though the Syrian civil war still rages on without an end in sight, the U.N. is trying to bring all the major actors involved to the table to begin the difficult task of establishing a transitional body to govern Syria.

The conference is a follow up to a similar summit in 2012, which called for the establishment of a transitional government and a democratic election. But international powers remain divided over how to achieve those goals.

Iran accepted a Sunday invitation by Ban to join the conference aimed at ending the bloody three-year civil between President Bashar Assad's government and opposition forces seeking his ouster. Ban has argued that Iran, because of its role as Assad's top backer, needs to be a part of the solution in Syria.

Iran's Foreign Minister Javad Zarif had privately pledged to play a “positive and constructive role,” the Associated Press reported.

The Iranian news agency ISNA on Monday, though, quoted a spokeswoman for Zarif saying it had accepted no terms or preconditions for attending the peace talks. Assad, in an interview with Agence France-Presse, also said he had no plans to relinquish or share power, in opposition to the aim of the upcoming international talks.

The invitation took the U.S. by surprise. Secretary of State John Kerry has said he would support Iran's inclusion in the talks only if the country supported the creation of a transitional government in Syria.

U.S. officials early Monday demanded that Ban rescind the invitation unless Iran fully agrees to the premise of the conference: that a power-sharing agreement, transitional government and free elections are necessary to end the civil war.

“The United States views the UN secretary general's invitation to Iran to attend the upcoming Geneva conference as conditioned on Iran's explicit and public support for the full implementation of the Geneva communique including the establishment of a transitional governing body by mutual consent with full executive authorities,” State Department spokeswoman Jen Psaki said in a statement. “This is something Iran has never done publicly and something we have long made clear is required.

“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,” she continued. “If Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded.”

A spokesman for Syria's opposition, which just a day earlier had decided to attend despite deep division among its rank about doing so, took to Twitter on Sunday threatening to pull out unless Ban “retracts Iran's invitation.”

This story was first published on Jan. 20 at 10:30 a.m.