BEIRUT — Syria’s main Western-backed opposition group said today that Iran must commit publicly by 7 p.m. GMT to withdraw its “troops and militias” from Syria and abide by a 2012 transitional roadmap, or else the U.N. should withdraw its invitation for Tehran to take part in a peace conference.

The Syrian National Coalition said if those conditions are not met by the deadline, then it will not attend the so-called Geneva 2 conference that is to begin Wednesday. The U.N. issued a last-minute invitation late Sunday to Iran, a close ally of Syrian President Bashar Assad, prompting the Coalition to threaten to sit out the peace talks and throwing the entire Geneva conference into doubt.

The negotiations are intended to bring together for the first time representatives of President Bashar Assad’s government and members of the Western-backed opposition that is trying to overthrow him. The conference aims to broker a political resolution to a conflict that has killed more than 130,000 people and touched off the worst humanitarian crisis in decades.