They will be the highest-level discussions between the foes since an international agreement on Iran's nuclear programme crumbled after Trump pulled out in 2018 during his first term in office.

Trump's special envoy Steve Witkoff and Iranian Foreign Minister Abbas Araghchi are set to lead the discussions behind closed doors in Muscat, the capital of Oman, which has long played a mediating role between Iran and Western countries.

Iranian state TV said that after his delegation arrived in Muscat on Saturday morning Araghchi met with Omani officials.

Hours before the talks were due to begin Trump told reporters aboard Air Force One: "I want Iran to be a wonderful, great, happy country. But they can't have a nuclear weapon."

Trump had made a surprise announcement last Monday that the talks would take place.

Iranian supreme leader Ayatollah Ali Khamenei's adviser Ali Shamkhani said Tehran was "seeking a real and fair agreement", adding that "important and implementable proposals are ready".

If Washington showed goodwill, the path forward would be "smooth", he said on social media platform X.

The format for the talks has not been confirmed, with the United States calling them direct talks but Iran insisting on an intermediary.

According to Iranian news agency Tasnim, the delegations will start indirect negotiations after a meeting with Omani Foreign Minister Badr bin Hamad Al Busaidi.

The talks are expected to begin in the afternoon with Busaidi as intermediary, Tasnim added.

Trump announced the talks during a White House press appearance with Prime Minister Benjamin Netanyahu of Israel, Iran's arch foe.

- 'Red line' -

The contact between the two sides, which have not had diplomatic relations for decades, follows repeated threats of military action by both the United States and Israel.

"If it requires military, we're going to have military," Trump said on Wednesday when asked what would happen if the talks fail to produce a deal.

Responding to Trump's threat, Tehran said it could expel United Nations nuclear inspectors, a move that Washington warned would be an "escalation".

Iran, weighed down by years of sanctions and weakened by Israel's pummelling of its allies Hezbollah in Lebanon and Hamas in Gaza, has strong incentives to negotiate.

The United States wants to stop Iran from ever getting close to developing a nuclear bomb.

The 2015 deal which Trump withdrew from aimed to render it practically impossible for Iran to build an atomic bomb, while at the same time allowing it to pursue a civil nuclear programme.

Britain, China, France, Russia and Germany were the other parties to the agreement, of which Araghchi was a key architect.

Witkoff told The Wall Street Journal that "our position today" starts with demanding that Iran completely dismantle its nuclear programme -- a view held by hardliners around Trump that few expect Iran would ever accept.

"That doesn't mean, by the way, that at the margin we're not going to find other ways to find compromise between the two countries," Witkoff told the newspaper.

"Where our red line will be, there can't be weaponisation of your nuclear capability," added Witkoff.

Iran, which insists its nuclear programme is only for civilian purposes, stepped up its activities after Trump abandoned the 2015 agreement.

The latest International Atomic Energy Agency report noted with "serious concern" that Iran had an estimated 274.8 kilograms of uranium enriched to 60 percent, nearing the weapons grade of 90 percent.

- 'Survival of the regime' -

Karim Bitar, a Middle East Studies lecturer at Sciences Po university in Paris, said negotiations "will not focus exclusively on... the nuclear programme".

"The deal would have to include Iran stopping its support to its regional allies," a long-standing demand by US allies in the Gulf, he said.

For Iran, it could be a matter of the government's very survival.

"The one and only priority is the survival of the regime, and ideally, to get some oxygen, some sanctions relief, to get their economy going again, because the regime has become quite unpopular," Bitar said.

Ali Vaez, of the International Crisis Group think-tank, said agreeing the scope of the talks would be "one of the first and most consequential issues".

"Iran does not want an expanded agenda in the early stages. But no deal will be sustainable unless it becomes more comprehensive," he said.

Iran is "likely to engage on steps to roll back its nuclear programme, but not dismantle it entirely" in exchange for sanctions relief, Vaez added.