The United States urged arch foe Iran on Sunday to back calls for a transitional government in Syria, or lose an invitation to international talks about the crisis.
UN Secretary-General Ban Ki-moon invited Iran to a peace conference beginning Wednesday, following the world body chief's intensive talks over two days with Iranian Foreign Minister Mohammad Javad Zarif, who agreed to attend.
"The United States views the UN secretary-general's invitation to Iran to attend the upcoming Geneva conference as conditioned on Iran's explicit and public support for the full implementation of the Geneva communique," US State Department spokeswoman Jen Psaki said in a statement.
"This is something Iran has never done publicly and something we have long made clear is required."
She noted that the statement adopted by the major powers in Geneva on June 30, 2012 called for establishing a Syrian transitional governing body by mutual consent with full executive authorities.
The United States and other Western powers have long opposed Iran's attendance at the meeting as long as it refused to accept the communique.
Up to this weekend, Iran had said it would attend the peace conference as long as there were no conditions.
"We also remain deeply concerned about Iran's contributions to the Assad regime's brutal campaign against its own people, which has contributed to the growth of extremism and instability in the region," Psaki said.
"If Iran does not fully and publicly accept the Geneva communique, the invitation must be rescinded."
If Iran attends, all key countries linked to the worsening war would be among 40 nations at the opening of the peace conference in Montreux, Switzerland on Wednesday.