A high-level IAEA delegation is to visit Iran January 29-31 to discuss issues it has over Tehran's nuclear programme, Iran's envoy to the UN atomic watchdog said Tuesday according to the Fars news agency.

The delegation, to be led by chief inspector Herman Nackaerts, will "negotiate and discuss questions raised by the IAEA," Ali Ashgar Soltanieh was quoted as saying.

Diplomats in Vienna, where the International Atomic Energy Agency is headquartered, had already said Nackaerts and the agency's number two, Rafael Grossi, were going to visit Iran late this month.

They had spoken of a visit starting around January 28 and lasting into the first few days of February.

Soltanieh said the three-day visit finally agreed was "a new sign of the transparency in Iran's nuclear programme and activities and in our interaction with the International Atomic Energy Agency."

The trip comes at a moment of high international tensions over Iran's nuclear programme.

The IAEA issued a report in November expressing strong suspicions -- but stopping just short of declaring them verified -- that Iran was researching the development and delivery of nuclear weapons.

The West, the United States at the fore, has subsequently ratcheted up sanctions on Iran, threatening its ability to export and get paid for its oil.

The European Union is expected next week to announce additional measures said to include a ban on Iranian oil imports.

IAEA diplomats said that, along with Nakaerts, who is Belgian, and Grossi, an Argentine who is IAEA head Yukiya Amano's chief of staff, the delegation will include the agency's senior legal official Peri Lynne Johnson, a US citizen.

"The aim of this mission is to try to get answers once and for all to all the questions raised by the IAEA's report in November," one diplomat told AFP on condition of anonymity.

Iran denies seeking atomic weapons, saying its programme is peaceful, but Western countries strongly suspect otherwise, and the UN Security Council has slapped four rounds of sanctions on the Islamic republic.

Last week, the IAEA said Iran had starting enriching uranium to purities approaching that needed for a nuclear weapon inside a mountain bunker at Fordo, near the holy city of Qom.

Iran says the 20-percent enriched uranium is for medical purposes. But Washington has called the start of operations at Fordo "a further escalation of their ongoing violations with regard to their nuclear obligations."