U.N. Secretary-General Ban Ki-moon said on Sunday he had invited Iran to attend the first day of Syria peace talks on January 22 in Montreux, Switzerland and that Tehran had pledged to play a "positive and constructive role" if it was asked to participate.
More than 100,000 people have been killed and over 2 million have fled Syria during the nearly three-year civil war.
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Western and Gulf Arab nations say they have been reluctant to the support the idea of Iran participating at all because it is supporting Syrian President Bashar al-Assad militarily and has never backed a plan for a political transition in Syria agreed at an international conference in Geneva in June 2012.
Ban said he had spoken at length with Iran's Foreign Minister Mohammad Javad Zarif in recent days and that he believed Tehran supported for the Geneva 2012 plan.
"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 communique," he said.
"Foreign Minister Zarif and I agreed that the goal of the negotiations is to establish by mutual consent a transitional governing body with a 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."
"Therefore as convener and host of the conference I have decided to issue an invitation to Iran to participate," Ban said. "Iran needs to participate as one of the important neighboring countries."
Ban said he expected Iran would issue a statement soon in response to his invitation.
Earlier this year the United States said that Iran might play a role on the sidelines of the Syria peace conference in Montreux. Tehran dismissed the idea of being relegated to the sidelines, saying it was beneath Iran's dignity.
The key players in the talks are Assad's government and opposition rebels who have been fighting for nearly three years to oust him.
Ban said he had invited on Sunday a total of 10 additional countries to attend on January 22 - the Vatican, Australia, Bahrain, Belgium, Greece, Luxembourg, Mexico, the Netherlands, South Korea and Iran. Speaking to reporters at U.N. headquarters, Ban made clear that the full negotiations between the government and opposition would begin in earnest on January 24 in Geneva.
Syrian opposition groups and Washington, which accuse Tehran of supporting Assad with manpower and arms during the uprising against him, have long had reservations about the participation of Iran, although Ban and the United Nations' special envoy on Syria, Lakhdar Brahimi, have long backed Tehran's involvement.
While there has been a warming in U.S.-Iranian ties this year including a November 24 deal to curb the Iranian nuclear program, there are no visible signs that this has led to greater improvement in other areas such as Syria, where they are on opposite sides of the civil war.
The idea of what U.N. diplomats refer to as the "Geneva 2" peace conference on Syria was first proposed by Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry last spring, though it has taken nearly a year to organize.
(Reporting by Michelle Nichols and Louis Charbonneau; Editing by Eric Walsh)