The UN atomic watchdog is concerned that Tehran may be working on a nuclear warhead, according to a restricted report obtained by AFP Thursday.

"The information available to the agency ... raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile," the watchdog's chief Yukiya Amano wrote in his first report to its board of governors.

The language of the report was much more blunt than that used by Amano's predecessor Egyptian Mohamed ElBaradei, who stepped down as head of the International Atomic Energy Agency (IAEA) at the end of November.

The Vienna-based IAEA has been investigating for a number of years intelligence reports claiming Iran was involved in weapons research.

These so-called "alleged studies" included uranium conversion, high explosives testing and the adaptation of a ballistic missile cone to carry a nuclear warhead.

A US intelligence report in 2007 said Iran halted such research in 2003, but Amano's report gives credence to the belief held by some western countries that the programme continued.

The information was "extensive ... broadly consistent and credible in terms of the technical detail, the time frame in which the activities were conducted and the people and organizations involved," the report said.

The 10-page document, which is to be discussed by IAEA governors at a meeting next month, also confirmed Tehran had begun enriching uranium to higher levels, theoretically bringing it closer to the levels needed for an atomic bomb.

Iran has previously reached uranium enrichment levels of no more than five percent at its facility at Natanz, in defiance of UN orders for it to cease and despite three rounds of UN sanctions.

Earlier this month, Iran announced it would begin enriching uranium to 20 percent, ostensibly to make the fuel for a research reactor that makes medical radioisotopes.

Iran insists its intentions are peaceful, but western powers suspect Tehran is enriching uranium to make nuclear weapons, as the material in high purity form can be used in the core of a atomic bomb.

The report said while the Islamic republic officially informed the IAEA of its intentions, it started feeding nuclear material into the uranium-enriching centrifuges before IAEA inspectors arrived in the plant to oversee the process.

The report said Iran had moved most of its stockpile of low-enriched uranium -- 1,950 kilograms from an estimated total of 2,065 kilograms -- for processing to higher levels.

Amano complained that Iran was continuing to stall agency requests to clear up the alleged weapons research.

"Since August 2008, Iran has declined to discuss the above issues ... or provide any further information and access to locations and people to address these concerns," the report said.

Iran has simply dismissed such allegations as "baseless" and the intelligence on which they were based as "forged".

IAEA inspectors verified that none of Iran's declared nuclear material had been diverted, the report said.

But it also said: "Iran has not provided the necessary cooperation to permit the agency to confirm that all nuclear material in Iran is in peaceful activities."

The IAEA urged Iran to cooperate fully and allow its inspectors access to all relevant sites, equipment, documentation and personnel "without further delay."

Only by doing so would the IAEA be able to make progress.

Iran's envoy to the IAEA, Ali Asghar Soltanieh, ignored Amano's concerns, insisting that the report "confirms Iran's nuclear activities are peaceful and that there is no deviation of material and programme for military purposes."

"In reality he (Amano) has once again confirmed all of the past six years (of IAEA's) reports that Iran's nuclear activities have been a peaceful one," Soltanieh was quoted by IRNA news agency as saying.

The United States voiced renewed concern about Iran's nuclear programme, with State Department spokesman P.J.Crowley telling reporters: "We cannot explain why it (Iran) refuses to come to the table and engage constructively to answer the questions that have been raised."