The United States said Sunday that a UN invitation to Iran to participate in this week's Syria peace conference in Geneva should be rescinded unless Tehran "fully and publicly" accepts the Geneva Communique.

The invitation should be withdrawn unless Tehran publicly states its support for the full implementation of the Geneva Communique signed in 2012, which agreed to establish a transitional governing body in Syria, U.S. State Department spokeswoman Jen Psaki said in a statement.

The precondition is "something Iran has never done publicly and something we have long made clear," Psaki said, adding that the U.S. remained "deeply concerned" about Iran's support to the Syrian government.

UN Secretary-General Ban Ki-moon said Sunday that he has invited Iran, a main regional ally of the Syrian government, to attend the Geneva II peace conference on Syria.

"I believe strongly that Iran needs to be part of the solution to the Syrian crisis," Ban said at a press conference.

He said that Iranian Foreign Minister Mohammad Javad Zarif has assured him that "Iran understands that the basis of the talks is the full implementation of the June 30, 2012 Geneva Communique."?