Witkoff, a friend who has served as President Donald Trump's envoy on both the Gaza and Ukraine wars, will take part in the talks Saturday on Oman, State Department spokeswoman Tammy Bruce told reporters.

Setting a tough line ahead of the talks, Bruce said: "On Saturday, there's a meeting. There's no negotiations."

Trump "is committed to diplomacy," she added.

"He's spoken about wanting to have a deal with Iran to get this done. And now it's up to Iran -- and if they don't want to, it will be very, very bad for them," Bruce said.

The president believes that "doing a deal would be preferable to doing the obvious," she said, in a clear reference to a military attack.

Bruce declined to go into depth on the US message in Oman other than saying that the United States believed that "Iran can never obtain a nuclear weapon."

Iranian Foreign Minister Abbas Araghchi has said he believes that the clerical state can reach a deal with the United States, its arch-enemy, if Washington shows goodwill.