UN leader Ban Ki-moon battled Monday to save a Syria peace conference after the Syrian opposition threatened to withdraw if Iran takes part.
Ban's spokesman said he was "dismayed" by Iran's refusal to back an international statement calling for a transitional government in Syria and the Syrian opposition's threat to boycott the talks.
"The secretary general is currently urgently considering his options in light of the disappointing reaction of some participants," UN spokesman Martin Nesirky told reporters.
UPDATE: Ban withdraws invitation »
Nesirky said Ban is in contact with the US and Russian foreign ministers over the mounting storm just two days ahead of the start of the peace conference in the Swiss town of Montreux.
He would not confirm that Ban would leave for Montreux later Monday.
Ban "is dismayed by the developments regarding participation in the Geneva conference," Nesirky said.
"Iran, despite assurances provided orally to the secretary general, has made a disappointing public statement that suggests Iran does not accept" a statement agreed by the major powers in 2012 as the basis for the peace conference.
The Geneva declaration of June 30, 2012 calls for the creation of a transitional governing body in Syria with full executive powers.
President Bashar al-Assad, who gets military and financial support from Iran, has, however, rejected any suggestion he should stand down.
The Syrian National Coalition, the main opposition group, set Ban a deadline of 1900 GMT to withdraw his invitation to Iran or it would not attend the meeting, it announced on its Twitter account.
The United States and European powers all demanded Iran "explicitly" support the call for a transitional government.
US ambassador to the United Nations, Samantha Power, said that a public statement by Iran was "a minimum requirement for participation in this peace process."
Britain and France also believe that Iran must publicly accept the Geneva communique before it takes part, their UN ambassadors told reporters.
"For the moment, the ball is in Iran's court. Iran must explicitly accept Geneva I," said France's UN ambassador Gerard Araud.
Ban told the Security Council earlier that he was in "intensive and urgent" talks on the peace meeting.
The UN secretary general announced Sunday he had invited Iran to the first day of talks, unleashing a storm of controversy.
Ban had previously joined Russia in calling for Iran's attendance at the peace talks.
"I believe strongly that Iran needs to be part of the solution to the Syrian crisis," Ban said Sunday.
He added that Iran's Foreign Minister Javad Zarif had repeatedly assured him in talks at the weekend that Iran would "play a very positive and constructive role" in efforts to end the three year-old civil war.
The United States and Russia, also a key Assad backer, played a key role in persuading the government and opposition to attend the talks.
Russia's UN ambassador Vitaly Churkin said Ban had consulted with the Russian and US governments before issuing the invitation to Iran. "Of course everybody was consulted," Churkin told reporters.
Despite the US protests over the invitation, a senior US official said that US Secretary of State John Kerry had spoken with Ban at the weekend.
With extra invitations issued at the weekend, 40 countries were to be at the opening of the peace talks on Wednesday. The Syrian government and opposition were then to meet in Geneva from Friday for talks.
