The United States said Wednesday that Iran's absence from Syria peace talks would have no impact on the nuclear deal being brokered between Tehran and world powers.

Washington spent months laying the groundwork with Moscow and the United Nations for the Syria peace conference dubbed Geneva II, which opened Wednesday in the Swiss town of Montreux.

UN Secretary-General Ban Ki-moon on Sunday invited Iran to the talks but withdrew his invitation less than 24 hours later amid reservations from the United States and Syrian opposition groups.

State Department deputy spokeswoman Marie Harf described the diplomatic crisis triggered by the invitation as a "fiasco."

"We have been very clear and the Iranians, I think, have been clear, certainly in our discussions, that these are separate issues," she told reporters.

"The nuclear negotiations are one set of negotiations, and the discussion about if Iran should go to Geneva II or what role they can play in Syria are completely separate.

"We've clearly had credible disagreements over Syria throughout the entire nuclear negotiating process. That really is separate. I think both we and the Iranians probably have a desire to keep them separate," Harf added.

The US State Department had explicitly asked that Ban rescind his invitation, noting that Tehran had never accepted an agreement signed by world powers in June that called for a transitional government in Syria.

Despite more than three decades without diplomatic ties, American and Iranian officials broke the ice in recent months.

In exchange for Iran verifiably halting its suspect nuclear program, the United States and the European Union each announced an easing to sanctions as negotiated with world powers under an interim six-month deal that came into force Monday.