One diplomat told AFP: "It's a bit puzzling they came and gave" the document to inspectors from the International Atomic Energy Agency (IAEA).

Other diplomats said the IAEA inspectors found it in a stack of other unrelated papers that the Vienna-based UN nuclear watchdog had asked for.

But IAEA vice chairman Mohammad Saidi said Saturday in Tehran that by handing over the document Iran was showing its good faith, reporting with "complete transparence" on its controversial nuclear program.

Iran said the document was part of a 1987 offer from a black market network that it never acted upon.

The text spells out "procedural requirements for ... the casting and machining of enriched, natural and depleted uranium metal into hemispherical forms," the IAEA said in a confidential report released Friday to its 35-nation board of governors and obtained by AFP.

Gary Samore, a non-proliferation expert who was an official in former president Bill Clinton's White House, told AFP: "There is no other purpose for manufacturing highly enriched uranium in hemispheres except for nuclear weapons."

The disclosure of the document raised concerns about Iran's nuclear program, which Tehran says is a peaceful drive to produce electricity but the United States and Europe fear could conceal a secret program to develop atomic weapons.

A diplomat close to the IAEA said the document was found in "two or three cardboard boxes" full of papers which Iran handed over and was in a binder and consisted of some 10 pages.

The diplomat said the document was a "step-by-step" guide for turning uranium gas into enriched uranium metal and casting it into a hemispherical shape.

Samore, speaking from Chicago where he works for the MacArthur Foundation, said: "The first step is you have to convert the gas to metal, then melt and cast the metal, not too much at a time because you don't want a criticality accident (an explosion).

"Then you make it into a rough cast hemisphere, put it on a lathe and cut it to exact specifications," he said.

The diplomat close to the IAEA said the Iranian document had "a couple of drawings" but was by no means a blueprint and that there were "no drawings for the core" of a bomb where the hemispheres would lie.

Also, the document "doesn't give anything like dimensions," the diplomat said.

But the 1987 offer from the smuggling network run by disgraced Pakistani atomic scientist Abdul Qadeer Khan did provide blueprints for centrifuges, the IAEA has said.

The diplomat said the Iranian document was the same sort of texts and drawings the Khan network handed to Libya and South Africa but that no one has seen a "whole set" of the Khan documents detailing nuclear technology and materials.

"Khan and company were hawking the whole works" of bomb technology, a second diplomat said, while the first diplomat said the IAEA was "still working to find out what else Iran may have received."

Gregory Schulte, the US ambassador to the IAEA, told reporters Friday: "Iran owes the (IAEA) board an explanation why it had these documents, what it has done with them, and why it didn't disclose them in the past."

He said the "documents open new concern about weaponization that Iran has failed to address."

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