United Nations Secretary-General Ban Ki-moon says "intensive and urgent discussions are under way" around his invitation for Iran to join this week's peace talks on Syria.
Mr. Ban told the U.N. Security Council Monday that he will have more to say on the subject later in the day. He announced the Iran invitation Sunday evening.
Senior U.S. officials said Monday the invitation must be withdrawn unless Iran fully endorses the formation of a transitional government for Syria that would pave the way for democratic elections.
Syria's main Western-backed opposition grouping said Iran must commit publicly by 1900 GMT to withdraw its "troops and militias" from Syria and abide by those terms, or the U.N. 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.
Mr. Ban indicated his invitation to Tehran came after he had received assurances from Iranian Foreign Minister Javad Zarif that Iran had accepted that premise.
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 U.S. 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.