The report, in German weekly Stern, cited BND experts as saying Iran had mastered the enrichment technology necessary to make a bomb and had enough centrifuges to make weaponised uranium.

It quoted one expert at the agency as saying: "If they wanted to, they could detonate an atomic bomb in half a year's time."

But a BND spokesman said the article did not reflect the view of the agency, which is that Iran would not be able to produce an atomic bomb for years.

"We are talking about several years not several months," the spokesman said.

Iran says its nuclear programme is for electricity generation to help it export more of its oil and gas, but Western countries suspect it of trying to make a nuclear bomb.

"(Six months) is absolutely a worst-case analysis," said Mark Fitzpatrick, senior non-proliferation fellow at London's International Institute for Strategic Studies.

He said that while it might be plausible in theory that Iran could further enrich uranium in a large enough quantity for a bomb as well as restarting the weapon design work it halted in 2003, these actions would not go unnoticed.

He said there was also disagreement as to how advanced the weapons design work was.

"If Iran were to go for broke and produce a nuclear weapon in this manner, it would have to expel International Atomic Energy Agency inspectors and the world would know," he said.

The U.N. Security Council has imposed three sets of sanctions on Tehran for defying its demands to suspend uranium enrichment.

Some analysts say Iran may be close to having the required material for producing a bomb, but most say the weaponisation process would then take one to two years due to technical and political hurdles.

Until now there have been no indications of any such covert diversion, a point made by the IAEA's incoming director-general shortly after his election earlier this month.

Current IAEA chief Mohamed ElBaradei has said it is his "gut feeling" that Iran is seeking at least the capability to build nuclear weapons, in order to protect itself from perceived regional and U.S. threats.

The Islamic Republic has largely rebuffed efforts by U.S. President Barack Obama for dialogue and has sharpened its rhetoric against the West following its disputed presidential election in June.