Syria's main, Western-backed opposition has said it will not attend UN-hosted peace talks in Geneva aimed at ending the country's civil war if Iran joins the gathering.
Ahmad Ramadan, a senior member of the Syrian National Coalition, said the opposition is "suspending" its participation because Iran is "invading" Syria.
Iran is president Bashar Assad’s top regional ally.
Today's comments by Mr Ramadan came hours after the United Nations invited Iran to attend the international meeting of foreign ministers in the Swiss city of Montreux ahead of the first direct peace talks between the warring Syrian sides.
UN chief Ban Ki-Moon said he invited Iran after lengthy talks with Iranian Foreign Minister Javid Zarif.
Mr Ramadan said “if the situation does not change, the coalition will not be” at the talks this week.
Mr Ban said he issued the invitation to Iran after “speaking at length in recent days” with Mr Zarif, who had “pledged that Iran would play a positive and constructive role in Montreux”.
The aim of the conference, dubbed Geneva 2, is to agree on a roadmap for Syria based on one adopted by the US, Russia and other major powers in June 2012. That plan includes the creation of a transitional government and eventual elections.
Iran is Assad’s strongest regional ally, extending him billions of dollars in credit since the crisis began in March 2011.
The US, Saudi Arabia and several countries in the Persian Gulf suspect Tehran is also shipping him weapons.
"We informed Ban Ki-Moon in the past that the coalition will not attend Genva 2 if Iran was invited," Mr Ramadan said by telephone from Istanbul.
"We consider Iran a country that is invading Syria and sending militias, whether it's Revolutionary Guards or Hezbollah. "
In 2012, the chief commander of Iran's powerful Revolutionary Guard, General Mohammad Ali Jafari, said the unit had high-level advisers in Syria but denied it has fighters there.
More recently, however, analysts say that Iranian troops and commanders have taken on a more direct role in the conflict. Lebanon’s Iran-backed Hezbollah fighters openly joined the war in Syria last year fighting along with Assad’s forces.
It remained unclear how definitive the Syrian opposition pullout was and how far it would go to get Iran away from Geneva.