UN investigators want Iran to explain an organisational chart linking projects to process uranium, test explosives and modify a missile cone for a nuclear payload, diplomats briefed on the matter say.

They said a top UN nuclear watchdog official last week gave a detailed presentation of intelligence alleging illicit atomic "weaponisation studies" by Iran and naming the man who ran them for the Ministry of Defence and Armed Forces Logistics.

In a written summary given to Reuters of the presentation, they said Iran had refused to let inspectors interview Mohsen Fakrizadeh or visit sites where the experiments took place.

The summary also confirmed leaks that the briefing for the first time indicated Iran continued the three projects into 2004, calling into question a US intelligence estimate in December that said Iran shelved weaponisation research in 2003.

"This presentation was a graphic demonstration that ...amplifies the concerns we've had for a number for years. And we are waiting for answers," Simon Smith, British ambassador to the IAEA, told reporters after the February  25 briefing.

The disclosures came as the United States and key European allies were piling pressure on four developing nations on the UN Security Council to vote for sanctions against Iran on Monday for refusing to halt its uranium enrichment programme.

Iran says its nuclear ambitions are limited to the peaceful generation of electricity and has dismissed the intelligence, key bits from a laptop smuggled out of the Islamic Republic and passed to Washington, as baseless, forged or irrelevant.

The Vienna-based IAEA says it remains to be seen whether the new intelligence details are correct, but is demanding a full response from Iran, not just denials lacking evidence.

Fakrizadeh, a military officer, earlier headed a military-affiliated physics research centre that was razed in 2004 after the IAEA asked to inspect it for signs of undeclared nuclear research.

In the power-point presentation, IAEA safeguards chief Olli Heinonen displayed an organisational diagram linking the three projects with numbered code names -- "5" for processing nuclear fuel, "110" for purported tests of an atomic device and "111" for a longer-range, Shahab-3 missile adapted to carry it.

Project 111 was also known as the "Orchid Office".

One of dozens of slides screened by Heinonen cited a progress report on the related projects for the period July 9, 2003-January 14, 2004. Other files showed the warhead design project began in July 2002.

US spy services estimated Iran halted outright "weaponisation" work in 2003 but also said it continued efforts to master technology applicable to yielding nuclear explosives.

The summary said Heinonen showed diagrams depicting tests with explosives to be placed in a shaft 400 metres (1,300 feet) underground and detonated from 10 km (6 miles) away.

Electrical bridge-wire (EBW) detonators would be used to ensure the several fissile layers of the warhead blew up in a chain reaction within 130 nanoseconds.

"The high-tension firing systems and multiple EBW detonators fired simultaneously are key components of nuclear weapons," the summary quoted Heinonen as saying.

Iran had said its explosives tests were for conventional arms only, he told the diplomats from the IAEA's 35-nation board of governors who will debate Iran at a meeting starting Monday.

Heinonen cited documentation from "Project 111" showing steps, including mathematical simulations, to design a "spherical warhead", suitable for the Shahab-3 missile, that would explode at a height of 600 metres (2,000 feet).

He said that altitude excluded the possibility that the warheads would be for chemical or biological weapons.

Diplomats quoted Heinonen as saying the IAEA needed more information before it could judge whether the intelligence was genuin