The United States has briefed key nations on intelligence that it says shows Iranian atomic weapons work, namely research on getting a missile warhead to explode at an altitude that would maximise the blast of a nuclear explosion, diplomats and analysts told AFP.
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," namely 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 cone 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 criticised 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 Russia, China, India, South Africa, as well as Ghana and Mexico, which are on the IAEA's 35-nation board of governors, diplomats said.