The UN chief has invited Iran, one of Syrian President Bashar al-Assad's main regional allies, to participate in the first phase of the Syria peace conference.
Making the announcement on Sunday at the UN headquarters in New York, Ban Ki-moon, UN secretary-general, said he invited Iran after Mohammad Javad Zarif, Iran's foreign minister, gave his backing to the peace talks in Switzerland and Iran pledged to play a "positive and constructive role" if it was asked to participate.
Shortly afterwards, the Turkey-based National Coalition of Syrian Revolution and Opposition Forces said it would withdraw from the conference unless Ban retracted his invitation.
The US also said the UN invitation should be withdrawn unless Iran publicly states its support for a 2012 agreement that establishes a transition government in Syria.
"If Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded," Jen Psaki, State Department spokeswoman, said in a statement.
Earlier on Sunday, the National Coalition met in Istanbul to appoint the members of a delegation heading to the peace conference.
Ban, in his remarks on Sunday, said Iran was among 10 additional countries invited to attend the Montreux meeting that would precede the talks scheduled to begin on Friday between Assad's delegation and Syrian opposition groups at the UN headquarters in Geneva.
Thirty other countries had already accepted invitations.
The US has long said that Iran should not be allowed to attend the talks in Switzerland until it states its support for a June 2012 agreement for a political transition in Syria.
However, Ban said Iran had agreed to endorse the principles calling for a transitional government as part of moves to end the ongoing civil war.
More than 100,000 people have been killed and more than two million have fled Syria during the nearly three-year strife.
Earlier on Sunday, Zarif said in Tehran that Iran would attend the Syria peace conference only if there were no preconditions for its participation.
"If the Islamic Republic [of Iran] is invited to this conference with no precondition, Iran will participate," he said.
Zarif said ignoring Iran's "outstanding role" in the region could be detrimental to the parties involved.
"The Islamic Republic has an outstanding role in this region, and those who do not want to use this unique role would do themselves a disservice and will deprive themselves of this outstanding role," he said.
The previous day, Iran's deputy foreign minister called on participants at the Syria peace conference to adopt a "realistic view" and decision that would not embolden armed fighters, according to media reports.
"Participants in the Geneva II forum must adopt a realistic view and remember that their decisions should not lead to the strengthening of extremist movements in Syria," Hossein Amir Abdollahian said.
He made the remarks during talks on Saturday with his French counterpart Jean-Francois Girault, Iran's news agency said.
"The forum can provide a political solution providing the people of Syria can decide the future of their country ... in a democratic solution that will be manifested in their votes," Abdollahian said.
Earlier this week, Sergey Lavrov, Russia's foreign minister, said Iran would inevitably become part of attempts to end Syria's civil war, and strongly urged the West to invite Iran to participate in the talks.