The UN nuclear watchdog said Tuesday it has received new information on possible military dimensions to Iran's nuclear programme, heightening concerns about the true nature of Tehran's atomic drive.

In a restricted new report, a copy of which was obtained by AFP, the International Atomic Energy Agency also said the Islamic republic has continued to increase its stockpile of low-enriched uranium, in defiance of multiple rounds of UN sanctions.

The IAEA has been investigating Tehran's atomic programme for a number of years now, which Iran insists are entirely peaceful.

But Western countries believe it masks a covert nuclear weapons programme and have compiled evidence that Iran was involved in weaponisation studies -- work which included uranium conversion, high explosives testing and the adaptation of a ballistic missile cone to carry a nuclear warhead -- at least until 2003.

In the new nine-page report, circulated to IAEA member states ahead of a board of governors meeting next month, the watchdog said it had "received further information related to such possible undisclosed nuclear-related activities, which is currently being assessed."

Furthermore, there were "indications that certain of these activities may have continued beyond 2004," the report said.

And a senior international official familiar with the investigation said the alleged activities in Iran may even have continued "until rather recently", including in 2010.

IAEA director general Yukiya Amano had written to Iran's vice president and head of its atomic energy body, Fereydoun Abasi, "reiterating its concerns (and) expressing the importance of Iran clarifying these issues," according to the report.

But Tehran has not yet responded, the official said.

Nevertheless, Amano's letter "makes it clear that we're very concerned and that Iran should engage with us," he said.

The IAEA calculated that Iran's stockpile of low-enriched uranium in the main branch of its Natanz uranium enrichment plant had now reached 4,105 kilogrammes.

Uranium enrichment is the most controversial part of Iran's nuclear activities because it can be used not only to generate nuclear fuel, but also to produce the fissile material for a nuclear bomb.

Tehran is under four sets of UN sanctions for refusing to halt such sensitive work.

But the Islamic republic remains defiant and has even started work on a second uranium enrichment facility at Qom.

In February, Tehran had said it planned to begin feeding nuclear material into cascades by the summer. But as of May 21, no uranium-enriching centrifuges had been introduced to the site, the IAEA said.

Last year, Iran also started enriching uranium to a higher level of purification, ostensibly for a research reactor that makes medical isotopes for cancer treatment.

The stockpile of 20 percent enriched uranium now amounts to 56.7 kilogrammes, the IAEA said.