Western countries including the United States have long suspected Iran of seeking to acquire nuclear weapons, an allegation Tehran has consistently denied, insisting that its programme is for peaceful civilian purposes.

"It's like a puzzle. They have the pieces, and one day they could eventually put them together," Grossi told French newspaper Le Monde in an interview published on Wednesday.

"There's still a way to go before they get there. But they're not far off, that has to be acknowledged," he said.

The UN watchdog was tasked with overseeing Iran's nuclear programme and its compliance with a 2015 nuclear deal which collapsed three years later when the United States withdrew from it in President Donald Trump's first term.

"It's not enough to tell the international community 'we don't have nuclear weapons' for them to believe you. We need to be able to verify," said Grossi, head of the International Atomic Energy Agency (IAEA).

He arrived on Wednesday in Tehran and met Foreign Minister Abbas Araghchi. He will also meet Mohammad Eslami, who heads Iran's nuclear energy agency.

Grossi's visit comes ahead of a second round of talks between Iran and the United States on Saturday, a week after the two countries held their highest-level talks since Trump abandoned the nuclear deal in 2018.

Both sides called the first meeting "constructive".

- 'Conflicting positions' -

Earlier, Araghchi said Iran's enrichment of uranium under its nuclear programme was "non-negotiable" after US Middle East envoy Steve Witkoff called for it to end.

"Iran's enrichment is a real, accepted matter," he told reporters.

"We are ready to build confidence in response to possible concerns, but the issue of enrichment is non-negotiable."

His remarks came after Witkoff said Tuesday Iran must "stop and eliminate" enriching uranium as part of any nuclear deal.

The day before, Witkoff had urged only that Iran return to the 3.67 percent enrichment ceiling set by its 2015 accord with major powers.

In its latest report, the IAEA said Iran had an estimated 274.8 kilos (605 pounds) of uranium enriched to up to 60 percent.

That level far exceeds the 3.67 percent ceiling set by the 2015 deal, but still falls short of the 90 percent threshold required for a nuclear warhead.

After returning to office in January, Trump reimposed sweeping sanctions against Iran under his policy of "maximum pressure".

In March, he wrote to Iran's supreme leader Ayatollah Ali Khamenei, urging talks but also warning of possible military action if these fail to produce a deal.

Ahead of Saturday's new round of talks, Araghchi condemned what he called the Trump administration's "contradictory and conflicting positions".

"We will find out the true opinions of the Americans during the negotiation session," he said.

Araghchi said he hoped to start negotiations on the framework of a possible agreement, but that this required "constructive positions" from the United States.

"If we continue to (hear) contradictory and conflicting positions, we are going to have problems," he warned.

- Iran's 'red lines' -

On Wednesday, Iranian state media said Saturday's talks will be in Rome with Omani mediation, as an Italian spokesman also confirmed the location.

However, US and Iranian officials have not officially confirmed the venue.

Araghchi is set to head to Iran's ally Russia on Thursday, Tehran's ambassador in Moscow Kazem Jalili said.

Iran has said the visit was "pre-planned", but will include discussions on the Iran-US talks.

"The objective of (my) trip to Russia is to convey a written message from the supreme leader" to President Vladimir Putin, Araghchi said.

In readiness for the US talks, Iran has engaged with Russia and China, which were both parties to the 2015 deal.

Ahead of Saturday's talks, Iranian President Masoud Pezeshkian said he hoped a deal could be reached with the United States, the official IRNA news agency reported.

On Tuesday, Khamenei cautioned that while the talks went well in their early stages, they could still prove fruitless.

"The negotiations may or may not yield results," he said, noting that Iran had already outlined its "red lines".

Iran's Revolutionary Guards have said the country's military capabilities are off-limits in the talks.

And late on Sunday, IRNA said Iran's regional influence and its missile capabilities -- both sources of Western concern -- were also among its "red lines".