However, a non-Western diplomat said the US briefing, carried out in various capitals ahead of a meeting in September of the UN atomic watchdog, "looks plausible but there is no hard evidence" giving direct proof of a nuclear warhead project.

Iran says its nuclear work is strictly peaceful and on Sunday hit back at the US allegations, with foreign ministry spokesman Hamid Reza Asefi saying: "It's a lie. It needs no more explanation."

A diplomat close to the Vienna-based watchdog International Atomic Energy Agency (IAEA) said that setting a warhead explosion at such a height, which is about 600 metres (yards), the same altitude at which the Hiroshima atomic bomb was detonated, would make sense only for nuclear weapons.

Chemical, biological or conventional weapons need to detonate closer to the ground in order to be effective.

The intelligence does not indicate whether the weapon the warhead is to hold is nuclear but the United States still considers the data the most important information it has on Iran, diplomats said.

The intelligence, the existence of which was first reported by the Wall Street Journal in March, contains diagnostic test information on putting a package, a so-called black box, inside the one of the medium-range Shahab-3 ballistic missile, a diplomat told AFP.

It consists of extensive Farsi-language computer files and reports. US officials are confident the data is genuine, diplomats said, even though some analysts have criticized it as unreliable since it is believed to come from only one source.

US officials in Vienna refused to comment on the matter.

A diplomat said that, according to the briefings, Iranian research was done from 2001-2003 at a semi-government owned industrial group that works on the Shahab missile and which was on a project commissioned by the elite Revolutionary Guards military.

The black box, actually a round container, is not identified as a nuclear warhead nor do blueprints show pits for uranium or plutonium, the two atomic bomb materials, but experts believe the package is meant to be atomic, diplomats said.

The United States gave the briefing to IAEA director general Mohamed ElBaradei and his deputy director for international safeguards Ollie Heinonen in July and then to several nations, including top allies such as Japan and EU states Britain, Germany and France, as well as Russia, China, India, South Africa, and Ghana, all on the IAEA's 35-nation board of governors, diplomats said.

The briefings ahead of an IAEA board meeting in September were part of campaigning for a resolution that found Iran in non-compliance with international nuclear safeguards and could lead to referring Tehran to the UN Security Council for possible sanctions.

Non-proliferation expert Gary Samore, a former US White House official under then-President Bill Clinton, described the data as "basically computer calculations of different configurations for a warhead delivery."

"I'm very confident that it's authentic," Samore said of the information, adding that it was "pretty clear that it was a nuclear warhead that was being designed."

ElBaradei, whose IAEA has been investigating Iran since February 2003, says "the jury is still out" on whether there is a covert atomic weapons program.

A Western diplomat said: "People are being careful because they have been burnt in the past," referring to faulty weapons intelligence about Iraq that was used to justify the US-led invasion of that country in March 2003.

But Washington-based non-proliferation expert David Albright, a physicist and former UN weapons inspector, said: "From my own knowledge of the documents, it appears to be a first effort to develop a credible re-entry vehicle for a nuclear weapon."

A diplomat said the program had the code-name Project 111.

Drawings for the warhead showed "a set of bridge wires," used to detonate explosives arranged in a circle to drive material inwards, the diplomat said.

Albright explained that this "very precise detonation of explosives creates a shock wave typically used to force a chain reaction in nuclear weapons."

The IAEA is trying to get access to certain military sites in Iran, including the Parchin facility where high-explosives work is carried out, as well as interviews with key people like Mohsen Fakrizadeh, who may be the head of the alleged Project 111, a diplomat said.

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