The 35-member board of governors of the International Atomic Energy Agency holds its regular summer board meeting until Friday.

Topping the agenda will be the latest report by IAEA Director General Mohamed ElBaradei on the agency's long-running investigation into Tehran's controversial nuclear drive.

Iran insists its atomic programme is entirely peaceful, but western countries, and the United States in particular, are convinced the Islamic republic is covertly seeking to build a nuclear bomb.

In the sternly-worded report, the IAEA expressed "serious concern" that Iran is hiding information about alleged weaponisation work, as well as defying UN demands to suspend uranium enrichment.

According to intelligence gathered by 10 different countries, Iran may have been looking into high explosives of the sort used in implosion-type nuclear bombs, and exploring modifications to missiles consistent with making them capable of delivering a nuclear weapon.

Iran has repeatedly dismissed the intelligence as fake and fabricated.

Nevertheless, in the report, the IAEA insisted that "substantive explanations are required from Iran."

The alleged weaponisation work "remain a matter of serious concern. Clarification of these is critical to an assessment of the nature of Iran's past and present nuclear programme."

In preparation for the board meeting, the agency's head of safeguards Olli Heinonen briefed diplomats on the technical aspects of the report.

According to diplomats who attended the meeting, Heinonen expressed "alarm" that Iran has in its possession a document describing the process for making what could be the core of a nuclear weapon.

The 15-page document describes the process of machining uranium metal into two hemispheres of the kind used in nuclear warheads.

Iran has told the IAEA that it received the document back in 1987 along with design information for the so-called P1 centrifuges used to enrich uranium.

Tehran insists it did not request the uranium metal document.

But the IAEA argues it needs to understand the precise role of the document to be able to determine the true nature of Iran's nuclear activities.

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