On Saturday, Araghchi held talks with US Middle East envoy Steve Witkoff in Muscat, the highest-level Iranian-US nuclear negotiations since the collapse of a 2015 accord.

"Dr Araghchi will travel to Moscow at the end of the week," said spokesman Esmaeil Baqaei, adding that the trip was "pre-planned" and would be "an opportunity to discuss the latest developments related to the Muscat talks."

Iran and the United States separately described Saturday's discussions as "constructive".

The negotiations came weeks after US President Donald Trump sent a letter to Iran's supreme leader Ayatollah Ali Khamenei calling for nuclear talks while warning of possible military action if Tehran refused.

Russia, a close ally of Iran, and China have held discussions with Iran in recent weeks over its nuclear programme.

Moscow welcomed the Iran-US talks as it pushed for a diplomatic solution and warned that military confrontation would be a "global catastrophe".

Another round of talks between Iran and the United States is scheduled for Saturday, April 19.

- 'Red lines' -

The official IRNA news agency reported that they would be held in Europe, without elaborating.

Baqaei said the next set of talks would continue to be indirect with Omani mediation, adding that direct talks were "not effective" and "not useful".

He had previously said that the only focus of the upcoming talks would be "the nuclear issue and the lifting of sanctions", and that Iran "will not have any talks with the American side on any other issue".

Late Sunday, IRNA reported that Tehran's regional influence and its missile capabilities were among its "red lines" in the talks.

In 2018, during Trump's first term in office, Washington withdrew from the 2015 agreement and reinstated biting sanctions on Tehran.

Iran continued to adhere to the agreement for a year after Trump's withdrawal but later began rolling back its compliance.

Iran has consistently denied that it is seeking nuclear weapons.

Baqaei reiterated that Iran will soon host United Nations nuclear watchdog chief Rafael Grossi but noted that the details of his trip were still "to be decided on."

Grossi, head of the International Atomic Energy Agency (IAEA), last visited Iran in November where he held talks with top officials, including President Masoud Pezeshkian.

In its latest quarterly report in February, the IAEA said Iran had an estimated 274.8 kilograms (605 pounds) of uranium enriched to up to 60 percent, close to the 90 percent level needed to make nuclear weapons.