Moderate Syrian rebels backed by the West threatened to pull out of this week's scheduled peace conference if the United Nations does not withdraw an invitation to Iran.

UN secretary-general Ban Ki-moon said that he had invited Iran to the talks in Montreux, Switzerland, after Tehran pledged to play a "positive and constructive role" in efforts to end Syria's three-year civil war.

But the Syrian National Coalition said furiously that it would withdraw from the negotiations unless the disputed invitation to Iran was retracted.

The United States warned that Iran must back a transitional government in Syria in conformity with the Geneva One decisions or forfeit the invitation.

Talks between President Bashar al-Assad's government and the opposition coalition of secularists, moderates and religious and ethic minorities had been due to start in Geneva on Friday.

But the meeting in Montreux to which Iran had been invited was more a "solidarity" gesture, said Mr Ban.

He told a news conference that he extended the late invitation to Tehran after two days of intense talks with Iranian Foreign Minister Mohammad Javad Zarif.

"Minister Zarif and I agree the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers," said Mr Ban.

Mr Ban, who had joined Russia in supporting Tehran's presence, said he expected a statement by the Iranian government.

"I believe strongly that Iran needs to be part of the solution to the Syrian crisis," he insisted.

He added that, as conference host, he had also extended last-minute invitations to Australia, Bahrain, Belgium, Greece, Luxembourg, Mexico, the Netherlands, South Korea and the Vatican.

But Syrian National Coalition member Ahmad Ramadan retorted that the opposition group was "suspending" its participation because Iran had forces in Syria and was "invading" the country.

"We informed Ban Ki-Moon in the past that the coalition would not attend Geneva Two if Iran was invited," Mr Ramadan said.

"We consider Iran a country that is invading Syria and sending militias, whether it's Revolutionary Guards or Hezbollah."

The threat of withdrawal came only hours after the coalition decided to join negotiations.