"Iran's enrichment is a real, accepted matter. We are ready to build confidence in response to possible concerns, but the issue of enrichment is non-negotiable," Araghchi told reporters after a cabinet meeting.

The remarks came as Araghchi and Witkoff are due to meet again in Oman on Saturday, a week after they held the highest-level talks between the longtime foes since US President Donald Trump abandoned a landmark nuclear deal in 2018.

Trump reimposed sweeping sanctions in a policy of "maximum pressure" against Tehran that he has reinstated since returning to office in January.

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

Both sides described Saturday's meeting as "constructive".

But on Tuesday, Witkoff said Iran must "stop and eliminate" its enrichment of uranium as part of any nuclear deal.

He had previously demanded only that Iran return to the 3.67 percent enrichment ceiling set by the 2015 accord between Iran and major powers that Trump withdrew from.

- 'Contradictory' -

Araghchi condemned what he called the "contradictory and conflicting positions" coming out of the Trump administration ahead of Saturday's talks.

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

Iran's top diplomat said he hoped to start negotiations on the framework of a possible agreement but said that required "constructive positions" from the United States.

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

Araghchi is set to head to Iranian ally Russia on Thursday, Iran'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 Russian 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 second round of talks in Muscat, 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 have proceeded 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.

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

The head of the International Atomic Energy Agency, Rafael Grossi was due in Iran later Wednesday for talks with senior officials.

The UN watchdog was tasked with overseeing Iran's compliance with the 2015 nulear deal.

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

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