A high-level UN nuclear agency delegation will visit Iran late this month to try to clear up claims of covert weapons activities, diplomats said Friday.

The trip led by International Atomic Energy Agency chief inspector Herman Nackaerts and the agency's number two Rafael Grossi would last from January 28 through the first week of February, one Western diplomat told AFP.

Another envoy also said the visit -- two months after an IAEA report on Iran took suspicions to a new level that Tehran is developing nuclear weapons -- would "likely" be from January 28, although it was not yet definite.

There was also some "ambiguity" on whether the delegation would merely hold talks with Iranian officials or be able to visit sites covered in the IAEA's damning November 8 report, the second diplomat said.

"It may be that the Iranians just want a short discussion in Tehran, which would not be what the IAEA is looking for," the envoy told AFP on condition of anonymity.

An IAEA spokesman declined to comment.

The delegation would include alongside the Belgian Nakaerts and the Argentine Grossi -- who is IAEA head Yukiya Amano's chief of staff -- the body's senior legal official Peri Lynne Johnson, a US citizen, envoys said.

"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," the first diplomat told AFP, without wishing to be identified.

Iran's ambassador to the agency, Ali Ashgar Soltanieh, told AFP late Friday that the details of the visit were still being worked out and that he hoped they would be "finalised" early next week.

"The visit will be made. We are in the final planning," he said, declining to comment on any details, including the timing of the trip, who would go, or whom or what the delegation would see.

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.

Ali Larijani, the influential speaker of Iran's parliament, said Thursday during a visit to Turkey that his country stood ready for negotiations with the five permanent members of the UN Security Council, plus Germany.

In its November report, rejected as "baseless" by Iran, the IAEA had said it was able to build an overall impression that Tehran "carried out activities relevant to the development of a nuclear explosive device."

Since the report, Western countries have sought to increase pressure on Iran, with Washington and Brussels taking aim at Iran's oil industry and central bank, while pressing Japan, China and others to join them.

Iran, where a judge on Monday reportedly sentenced to death a US-Iranian former Marine for "membership of the CIA", has threatened to close the Strait of Hormuz, a chokepoint for 20 percent of the world's oil.

Also on Monday the IAEA said that 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.

This was a "very significant step", Oliver Thraenert from the German Institute for International and Security Affairs (SWP) in Berlin told AFP, saying he was not optimistic that the upcoming IAEA visit would achieve much.

"We are already in a confrontation between the West ... and Iran, with more and more escalation going on on both sides," he told AFP. "The Iranians are becoming much more nervous, this is obvious."

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

On Wednesday Mostafa Ahmadi Roshan, a deputy director of Iran's main uranium enrichment plant, died in a car bomb blast that Tehran blamed on the US and Israel, the third scientists to meet such a fate in the past two years.