Syria's main opposition body, the National Coalition, will not attend peace talks in Switzerland scheduled for this week unless the United Nations retracts its invitation to Iran by 1900 GMT on Monday, a senior coalition member said.
Late on Sunday, UN Secretary-General Ban Ki-moon invited Iran to the conference, dubbed "Geneva 2", prompting the Coalition to quickly issue an ultimatum, on which it has now set a time limit.
"We are giving a deadline of 1900 GMT for the invitation to be withdrawn," Anas Abdah, member of the National Coalition's political committee, told Reuters.
Reading an official coalition statement, Abdah reiterated that the Coalition would accept Iran's participation only if it "publicly states that it is withdrawing its forces, committing to the Geneva 1 agreement in full and committing to implementing any results of Geneva 2".
The accord reached in Geneva in 2012 calls for a transitional government for Syria, which Western countries and the opposition say means Syrian President Bashar al-Assad must leave power.
Coalition members privately told Reuters that no one expected Iran, Assad's main backer, to meet any of these terms.
The National Coalition has been under immense pressure from the international community to attend the talks aimed at ending the conflict in which more than 100,000 people have been killed and millions displaced.
Over the weekend, following a series of delays, it finally said it would participate in the talks which are set to start on Wednesday in Montreux and are seen as the most serious international effort to end the near three-year conflict.
Iran's attendance has been one of the most hotly debated issues in the run-up to the conference, with Russia insisting that it would be a mistake if Iran did not take part.
Iran accepted the invitation on Monday "without preconditions".
There was no immediate comment from the United Nations or Iran on the ultimatum.