The UN nuclear watchdog is getting more and more worried about a possible military dimension to Iran's nuclear activities, according to a confidential report obtained by AFP Friday.

The International Atomic Energy Agency (IAEA) is "increasingly concerned about the possible existence in Iran of past or current undisclosed nuclear related activities involving military related organisations", the report said.

These included "activities related to the development of a nuclear payload for a missile", according to the report, which is due to be discussed by the IAEA's 35-member board of governors at a September 12-16 meeting.

The United Nations Security Council has slapped four rounds of sanctions on Iran to get it to suspend uranium enrichment, a process which can produce fuel for a reactor but which also can be used in a nuclear warhead.

The Islamic Republic began its 20-percent enrichment in February 2010, theoretically bringing it closer to the 90-percent level required to make an atomic bomb.

Tehran insists that its activities are aimed exclusively at developing nuclear power.

A diplomat told AFP on condition of anonymity that Iran was also making "lots of efforts" to get its Fordo enrichment plant, located deep inside a mountain near the Shiite shrine city of Qom, operational as soon as possible.

Revelation of the construction of the site 150 kilometres (95 miles) southwest of Tehran in 2009 infuriated the West and prompted the United Nations to strengthen sanctions against Iran.

Iran announced in June that it planned to triple its enrichment capacity when it transfers the work from its existing Natanz plant to Fordo later this year.

Since the IAEA's last quarterly report, Iran has provided "some clarification" on Fordo, but the agency still needs more information, the diplomat said.

Since February 2007, Iran has produced more than 4,500 kilos (9,920 pounds) of 3.5-percent enriched uranium (LEU) at its Natanz site plus around 70 kilos of 20-percent uranium since February 2010, according to the new IAEA report.

Last month a top IAEA official toured Iranian nuclear sites.

Herman Nackaerts, head of the IAEA's safeguards division, was able to visit over several days several sites, Ali Asghar Soltanieh, Iran's representative to the agency, told the FARS news agency.

These included the Bushehr nuclear power plant, Fordo and Natanz, installations at Isfahan including a nuclear fuel rod plant and the Arak heavy water research facility and production plant.

Tehran allowed the visit "to show the Islamic republic's goodwill", Soltanieh said.