UN Secretary-General Ban Ki-moon says he has invited Iran to participate in preliminary Syrian peace talks in Montreux, Switzerland.
Ban said that Tehran had pledged to play a "positive and constructive role" if it were asked to participate in the talks, which are due to start on January 22.
But a spokesman for Syria's main opposition group in exile said the Syrian National Coalition will pull out of the talks unless Ban retracts his invitation.
The reaction from the National Coalition came less than two days after the exiled opposition voted at talks in Turkey to participate in the peace negotiations.
U.S. State Department spokeswoman Jen Psaki said the invitation should be rescinded if Tehran does not fully accept principles of a previous peace conference calling for a transitional government in Syria.
Speaking to reporters at UN headquarters in New York on January 19, Ban said Iranian Foreign Minister Javad Zarif understands that the basis of the negotiations set to get underway in Switzerland is to set up a transitional government as spelled out in 2012's Geneva One declaration.
"Iran understands that the basis of this conference is the Geneva Communiqué One, and, the main goal of these Geneva Conference Two, should be to establish a transitional government body with full elective powers upon mutual consent," Ban said. "He (Iranian Foreign Minister Zarif) assured me, again and again that Iran, if they are invited, then they will play a very positive constructive role."
Ban also said Iran is among 10 additional countries invited to attend the Montreux meeting on January 22.
Two days later, formal negotiations between Syrian President Bashar al-Assad's delegation and Syrian opposition groups are scheduled to take place at UN headquarters in Geneva.
Invitations to the one-day meeting of foreign ministers at a Montreux hotel had been subject to approval by the initiating states, Russia and the United States, but the two countries had been at an impasse over whether Iran, Assad's strongest ally, should attend.
Speaking ahead of the discussions, Assad said there is a "good chance" he will seek a new term and ruled out sharing power with the opposition.
In an exclusive interview with the AFP news agency, which was published on January 20, Assad also said that the peace talks in Switzerland should focus on what he described as his "war on terrorism."
With reporting by AP, Reuters, and AFP