A UN atomic agency team visiting Iran from Sunday is highly unlikely to return with anything substantial enough to ease current tensions, experts including the IAEA's former chief inspector told AFP.

Iran's envoy to the International Atomic Energy Agency, Ali Asghar Soltanieh, said on Tuesday that Tehran hoped the three-day trip would "resolve any ambiguity and show (our) transparency and cooperation with the agency."

But the IAEA team led by chief inspector Herman Nackaerts and the Vienna-based body's number two, Rafael Grossi, will not be given access to any sites mentioned in a damning report by the agency in November, experts say.

"This is not a verification mission," Olli Heinonen, Nackaerts' predecessor and now at the Belfer Center for Science and International Affairs at the Harvard Kennedy School in the United States, told AFP.

"I don't think we should expect too much."

"My impression from the Iranian public statements is that this is talks about talks. If you look at who is going, it is not an overly technical team."

"I don't expect anything fundamental on the main issues," agreed Bruno Tertrais, senior research fellow at the Foundation for Strategic Research (FRS) in Paris.

Even if Nackaerts gets back to Vienna with Iranian promises of a new era of cooperation ringing in his ears, the list of previous false dawns -- and of occasions where Iran was less than open with the agency -- is long.

Mark Hibbs of the Carnegie Endowment for International Peace said that since the crisis began in 2003 there has been a pattern of Iran "coming up with some gambit ... whenever it is facing the threat of sanctions and punishment."

"It is conceivable that after the meeting with the IAEA, and after discussing the issues with the P5+1 (the five permanent members of the UN Security Council and Germany), that in the short term it could result in a commitment to come up with a roadmap for a long-term negotiation.

"But we haven't seen anything since 2003 that looks like that."

The IAEA "has been in and out of Iran for years and has yet to be fully satisfied with regard to Iran's programme," US State Department spokeswoman Victoria Nuland said this week.

"There is a lot of work to do. Obviously, one visit by itself ... by the IAEA after all this time can't constitute a complete substantive cooperation and transparency that we, the international community, the IAEA, are calling for."

The November report said its information, backed by more than 1,000 pages of documentation, intelligence from more than 10 countries and its own sources, "indicates that Iran has carried out activities relevant to the development of a nuclear device."

It detailed 12 suspicious areas such as testing explosives in a steel container at a military base and studies on Shahab-3 ballistic missile warheads that the IAEA said were "highly relevant to a nuclear weapon programme."

Iran, which has come under unprecedented international pressure since the publication of the report, with Washington and the EU targeting its oil sector and central bank, rejected the dossier as based on forgeries.

"This trip is aimed at neutralising enemy plots ... and baseless allegations, and proving the peaceful nature of our nuclear activities," Soltanieh told the official news agency IRNA on Tuesday.

What makes the international community all the more nervous is Tehran's continued defiance of UN Security Council resolutions calling on it to stop enriching uranium until the IAEA is satisfied its programme is peaceful.

Earlier this month the IAEA said Iran had begun enriching uranium to 20-percent purity deep inside a mountain bunker at Fordo, taking it significantly closer to the 90-percent mark needed for a nuclear bomb.

"In my view this (visit) is another opportunity for Iran now to start to come clean because having these questions on weaponisation unanswered, and then at the same time boosting their enrichment capabilities, is a combination which raises legitimate concerns," Heinonen said.