(Recasts with U.N. spokesman) By Louis Charbonneau and Michelle Nichols UNITED NATIONS, Jan 20 (Reuters) - The United Nations said on Monday that Secretary-General Ban Ki-moon is reviewing his options after Tehran's disappointing response to Ban's invitation to attend Syria peace talks and the Syrian opposition's threat to pull out if Iran attends. "The secretary-general was deeply disappointed by Iranian statements today that are not consistent with the assurances he received regarding Iranian support for the Geneva communique," Ban's spokesman Martin Nesirky told reporters, referring to a June 2012 plan for a political transition in Syria. "(Ban) is currently urgently considering his options in light of the disappointing reaction of some participants," Nesirky said, adding that Ban was also disappointed that the Syrian opposition had conditioned its participation on the withdrawal of Iran's invitation. An unexpected last-minute U.N. invitation for Iran to attend a peace conference on Syria in Montreux, Switzerland, this week threw the talks into doubt, with the Syrian opposition saying it would pull out unless Ban withdraws his offer. Earlier, Ban told the U.N. Security Council that "intensive and urgent discussions are underway and I'll have more to say about the situation later in the day." The Syria peace talks, widely referred to as "Geneva 2," are scheduled to begin on Jan 22. Iran and Russia are the main foreign backers of Syrian President Bashar al-Assad and Tehran's presence has been one of the most contentious issues looming over the first talks attended by both Assad's government and opponents, set to start on Wednesday. Nesirky said Iran's disappointing public statement was that it "does not accept that the basis for the Geneva conference ... is the full implementation of the 30 June, 2012 Geneva communique, including the establishment by mutual consent of a transitional governing body with full executive powers." He added that the Iranian statements were "not consistent with the assurances (Ban) received regarding Iranian support for the Geneva communique." Ban told reporters on Sunday that Iranian Foreign Minister Mohammad Javad Zarif indicated that Iran supported the Geneva communique from June 2012. DECISION TO INVITE IRAN 'NOT HASTY' Several Western diplomats told Reuters on condition of anonymity that Ban's decision to invite Iran appeared to have been made hastily and his advisers were now scrambling to prevent a collapse of the conference on Syria's nearly three-year civil war. Nesirky dismissed the allegation, saying countries like the United States were fully aware of the invitation to Iran before it was announced on Sunday. "This was not hasty," he said. "This could not have been a surprise to the U.S. authorities. It was not hasty and they were fully aware of the timing of the announcement." British U.N. Ambassador Mark Lyall Grant told reporters before the Security Council meeting that Iran must explicitly and publicly embrace a plan for a political transition in Syria that was agreed in June 2012 at an international conference in Geneva if it was to attend the so-called "Geneva 2" talks. The United States went a step further, saying it expects Ban to withdraw his invitation to Iran to attend Geneva 2 unless Tehran fully supports the June 2012 agreement on creating a transitional government for Syria - which Washington says would be without Assad. But Russian U.N. Ambassador Vitaly Churkin disputed suggestions that the decision to invite Iran was made hastily. Asked if Ban had consulted Russia and the United States in advance of announcing the invitation, Churkin told reporters: "Of course, everybody was consulted." He added that it would be "a big mistake" if the Syrian opposition boycotted the talks. (Reporting by Louis Charbonneau; editing by Chizu Nomiyama and Sofina Mirza-Reid)
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