The head of the Vienna-based International Atomic Energy Agency, the UN nuclear watchdog, will travel to Iran on Saturday "for meetings with senior Iranian officials", the IAEA said Thursday.

Director General Rafael Grossi will then hold a press conference on his return to Vienna, an agency spokesman said.

The announcement comes a day after Grossi vowed that the IAEA would "never abandon" its attempts to get Iran to clarify the previous presence of nuclear material at several undeclared sites there.

Iran has said the closure of the probe is necessary in order to clinch a deal to revive the 2015 deal with world powers on its nuclear programme.

The talks on the deal taking place in Vienna are widely seen as being at a crunch point, with the next few days key to their success or failure.

Diplomats from Britain, China, France, Germany, Iran and Russia restarted the talks in late November to revive the 2015 accord, also known as the JCPOA or Joint Comprehensive Plan of Action.

The US has been taking part indirectly.

The IAEA has been pressing Tehran for several years for explanations regarding indications that nuclear material was previously present at four different locations in Iran.

While much of the activity concerned is thought to date back to the early 2000s, sources say that one of the sites, in the Turquzabad district of Tehran, may have been used for storing uranium as late as the end of 2018.