The meeting between Mora and Iranian Deputy Vice President Ali Bagheri is "currently taking place", according to state news agency IRNA, which circulated pictures of the encounter without providing further details.

Talks between world powers and Iran have stalled since mid-March as negotiators seek to return to the landmark accord that curtailed the Islamic republic's nuclear programme in exchange for sanctions relief.

Mora has played a key role as an intermediary between the United States and Iran during a year of on-off talks in Vienna to restore the deal formally known as the Joint Comprehensive Plan of Action (JCPOA).

The deal was agreed in 2015 between Iran and the five permanent United Nations Security Council members Britain, China, France, Russia and the United States, plus Germany.

But Washington unilaterally withdrew in 2018 under then-president Donald Trump before imposing biting economic sanctions, prompting Iran to begin rolling back its own commitments.

Iran called in late April for a new meeting "as soon as possible" to move towards restoring the agreement.

Its foreign ministry spokesman, Saeed Khatibzadeh, said on Monday that Mora's visit "moves the talks in the right direction", but added this did not mean he would arrive with a "new message".