The U.N. nuclear watchdog has asked Tehran to explain evidence suggesting that Iranian scientists have experimented with an advanced secret nuclear warhead design, according to a report published Friday.

Citing what it calls "previously unpublished documentation" from an International Atomic Energy Agency compiled report, Britain's The Guardian newspaper said Iranian scientists may have tested high-explosive components of a "two-point implosion" device.

The report said that even the existence of two-point implosion nuclear warhead technology is officially secret in both the U.S. 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.

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

The U.N. watchdog's statements 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.

Extracts of the report have been published before, but it was not known the document included information on such a sophisticated warhead, the newspaper said.

A nuclear site, which Iran revealed in September three years after diplomats said Western spies first discovered it, added to fears of secret Iranian efforts to develop nuclear bombs. Iran claims it is enriching uranium only for peaceful electricity use.

The Vienna-based IAEA, Iran's Foreign Ministry and the Atomic Energy Organization of Iran were unavailable for comment when contacted by Reuters.

Continue reading at The Guardian

Reuters contributed to this report.