The U.N. nuclear watchdog chief said on Monday Iran seemed to be holding back information needed to clarify intelligence reports that it researched nuclear bombs in secret and he demanded "full disclosure" by Tehran.

In an unusually tough speech opening a meeting of the International Atomic Energy Agency's 35-nation Board of Governors, IAEA Director Mohamed ElBaradei said the intelligence remained a "matter of serious concern".

The Islamic Republic says the material, which was supplied by around 10 countries, primarily the United States, is forged but has not backed up its denials with evidence.