The two sides were engaged in "indirect" talks -- via an intermediary -- in the Omani capital Muscat, Iran's foreign ministry said. The Americans had called for the meetings to be face-to-face.

Disagreement over the format indicated the task facing the long-term adversaries, who are seeking a new nuclear deal after Trump pulled out of an earlier agreement during his first term in 2018.

Foreign Minister Abbas Araghchi is leading the Iranian delegation while Trump's special envoy Steve Witkoff heads the US team.

"Our intention is to reach a fair and honourable agreement from an equal position," Araghchi said earlier in a video posted by Iranian state television.

Iran's foreign ministry spokesman Esmaeil Baqaei told the broadcaster that the negotiations were "just a beginning" and that Tehran did not "expect this round of talks to be very long".

The two parties are in "separate halls" and "are conveying their views and positions to each other through the Omani foreign minister", he posted separately on X.

Iran, weakened by Israel's pummelling of its allies Hezbollah in Lebanon and Hamas in Gaza, is seeking relief from wide-ranging sanctions hobbling its economy.

Tehran has agreed to the meetings despite baulking at Trump's "maximum pressure" campaign of ramping up sanctions and repeated military threats.

Meanwhile the US, hand-in-glove with Iran's arch-enemy Israel, wants to stop Tehran from ever getting close to developing a nuclear bomb.

- Witkoff open to 'compromise' -

There were no visible signs of the high-level meeting at a luxury hotel in Muscat, where there were no flags or unusual security measures and little traffic on the streets.

Witkoff told The Wall Street Journal earlier 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 to 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," he added.

The talks were revealed in a surprise announcement by Trump during a White House appearance with Israeli Prime Minister Benjamin Netanyahu on Monday.

Hours before they began, 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."

Iranian supreme leader Ayatollah Ali Khamenei's adviser Ali Shamkhani said Tehran was "seeking a real and fair agreement".

Saturday's contact between the two sides, which have not had diplomatic relations for decades, follows repeated threats of military action by both the US 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.

- 'Survival of the regime' -

The multi-party 2015 deal that Trump abandoned aimed to make it practically impossible for Iran to build an atomic bomb, while at the same time allowing it to pursue a civil nuclear programme.

Iran, which insists its nuclear programme is only for civilian purposes, stepped up its activities after Trump withdrew from the 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.

Karim Bitar, a Middle East Studies lecturer at Sciences Po university in Paris, said a deal 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," he told AFP.

Mohamed al-Araimi, ex-head of the official Oman News Agency, said the highest-level talks since the last deal crumbled indicate "a strong desire to reach a resolution".

But he added: "Personally, I don't believe that today's meetings in Muscat will resolve all of these files. These matters require technical teams."