The UN nuclear watchdog has asked Iran to explain evidence suggesting the Islamic Republic's scientists have experimented with an advanced nuclear warhead design, the Guardian reported.

The newspaper, citing what it describes as 'previously unpublished documentation' from an International Atomic Energy Agency compiled dossier, said Iranian scientists may have tested high-explosive components of a 'two-point implosion' device.

The IAEA said in September it has no proof Iran has or once had a covert atomic bomb programme.

The Vienna-based IAEA was not immediately available for comment on Thursday.

Iran's Foreign Ministry and the Atomic Energy Organisation of Iran (AEOI) were also unavailable for comment when contacted by Reuters.

The IAEA statement in September followed reports from the Associated Press quoting what it called a classified IAEA document saying agency experts agreed Iran now had the means to build atomic bombs and was heading towards developing a missile system able to carry a nuclear warhead.

The Guardian report said that even the existence of two-point implosion nuclear warhead technology is officially secret in both the United States and Britain.

The technology allows for the production of smaller and simpler warheads, making it easier to put a warhead on a missile, the newspaper said.

Extracts of the dossier have been published before, but it was not known the dossier included documentation of such a sophisticated warhead, the newspaper said.

UN inspectors found 'nothing to be worried about' in a first look at a previously secret uranium enrichment site in Iran last month, IAEA chief Mohamed ElBaradei said in remarks released on Thursday.

ElBaradei also told the New York Times that he was examining possible compromises to unblock a draft nuclear cooperation deal between Iran and three major powers that has foundered over Iranian objections.

A nuclear site, which Iran revealed in September three years after diplomats said Western spies first detected it, added to fears of covert Iranian efforts to develop atom bombs. Iran says it is enriching uranium only for electricity. - Reuters