U.N. Secretary-General Ban Ki-moon is "urgently considering his options" in light of the "disappointing conduct of some participants" involved in the Syria peace talks after he invited Iran to attend.
U.N. spokesman Martin Nesirky told reporters Monday that Iran, despite assurances, "has made a disappointing public statement'' that suggests Tehran does not accept the terms of this week's peace talks in Switzerland.
"The statement made today in Tehran by the foreign ministry spokesperson fell short by some measure from what the secretary-general expected to hear."
Senior U.S. officials said Monday the invitation must be withdrawn unless Iran fully endorses a transitional government for Syria that would pave the way for democratic elections.
Syria's main Western-backed opposition group agreed, saying Iran must commit publicly by 1900 GMT to withdraw its "troops and militias" from Syria and abide by those terms, or the United Nations should withdraw the invitation.
The Syrian National Coalition also threatened to boycott the Geneva 2 peace talks - scheduled to open Wednesday in Switzerland - unless Mr. Ban retracts his decision to invite Iran.
France and Saudi Arabia joined the United States in rejecting Iran's participation in the talks unless Tehran publicly agreed to help to establish a transitional government for Syria with full executive powers.
But Nesirky said no one should have been surprised that Iran was asked to attend the meeting.
"I know for a fact this could not have been a surprise to U.S. authorities. [The decision to invite Iran] was not hasty and they were fully aware of the timing of the announcement. We have been in close contact with the Russians and the Americans today, over the weekend and for days and weeks before on precisely this topic. And with a variety of other leaders as well."
Mr. Ban indicated his invitation to Tehran came after he had received assurances that Iran accepted the purpose of the talks - to negotiate a transitional administration that would govern Syria by the "mutual consent" of President Bashar al-Assad's government and the opposition.
But Iran's ISNA news agency quoted Zarif's deputy Monday as saying Iran's participation did not mean it backed a plan for Syria's political transition agreed at a Geneva conference in June 2012.
Deputy Foreign Minister Hosein Amirabdollahian said acceptance of the so-called Geneva 1 agreement as a condition for attending Geneva 2 "is rejected and unacceptable." He said Iran would attend the talks "without any preconditions."
The United States and its allies say Iran has provided substantial military and economic support for Syrian President Bashar al-Assad and that the Geneva 1 plan means he would have to step down.
Mr. Assad told the French news agency in an interview published Monday there is a "significant" chance he will run again for president, and that he will not hesitate to do so if the public supports his candidacy.
Syria has said the issue of Mr. Assad giving up power is not up for discussion at the peace talks, so few experts expect the negotiations will reach this goal. But they say they do hope the discussions will result in increased humanitarian access and local cease-fires to make life easier for Syrian civilians.