Iran has been invited to attend a meeting of foreign ministers in Switzerland ahead of internationally brokered peace talks between Syria's warring factions, the United Nations said Sunday, a move that could potentially scuttle the war-torn nation's negotiations.

U.N. Secretary-General Ban Ki-Moon said he had issued the invitation to Iran after "speaking at length in recent days" with Iranian Foreign Minister Javid Zarif, who had "pledged that Iran would play a positive and constructive role in Montreux."

But the Syrian National Coalition, which was under huge pressure from its western and Arab sponsors to attend the peace talks, reportedly blasted the United Nations' announcement.

"The Syrian Coalition announces that they will withdraw their attendance in Geneva 2 unless Ban Ki-moon retracts Iran's invitation," National Coalition spokesman Louay Safi said in a Twitter post, Reuters reported.

U.S. Secretary of State John Kerry had said he would welcome Iran's participation — but only if Tehran endorsed earlier diplomatic agreements that called for a transitional government in Syria that would be created by mutual consent among the Syrian factions.

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On Sunday, a U.S. State Department spokeswoman echoed that stance.

"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," State Department spokeswoman Jen Psaski said in a statement. "If Iran does not fully and publicly accept the Geneva communiqué, the invitation must be rescinded."

Ban spoke to reporters at an impromptu briefing Sunday late Sunday afternoon. He said Iran is among 10 additional countries invited to attend the Montreux meeting that precedes the peace talks scheduled to begin Friday between Syrian President Bashar Assad's delegation and Syrian opposition groups at the U.N. European headquarters in Geneva. Thirty other countries had already been invited to attend the Montreux meeting.

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.

Ban said that Zarif had assured him that Iran "understands that the basis of the talks" is the full implementation of the road map adopted by the U.S., Russia and other major powers in Geneva in June 2012. That plan called for the creation of a transitional Syrian government with full executive powers.

"Foreign Minister Zarif and I agree that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers," Ban said. "It was on that basis that Foreign Minister Zarif pledged that Iran would play a positive and constructive role in Montreux."

The U.S. and Russia have been trying to hold the Geneva peace conference since last year and it has been repeatedly delayed.

Ban appealed to the Syrian parties "to keep one goal in mind: the end of the suffering of the Syrian people and the beginning of a transition to a new Syria."

Ban, who was scheduled to depart for Switzerland on Monday, said Montreux was "not a venue for negotiations" but intended as a gathering of countries and organizations "to show their solidarity" with the peace process and Syrian people.

The Geneva talks will be moderated by the U.N.-Arab League special envoy to Syria, Lakhdar Brahimi.

The last U.N. estimate in July put the death toll from the Syrian civil war at 100,000, though activists more recently gave a figure of 130,000. Along with 6.5 million internally displaced people, there are 2.3 million Syrians who have fled the country during the war.

The Associated Press contributed to this report.