Iran’s promise to clarify its use of detonators marks only an initial step by Tehran to address long-standing allegations of past nuclear weapons research.

Iran's promise to clarify its use of detonators marks only an initial step by Tehran to address long-standing allegations of past nuclear weapons research, the UN atomic watchdog said Monday.

"This is the first step that is taking place now," International Atomic Energy Agency chief inspector Tero Varjoranta told reporters at Vienna airport after returning from Iran.

"There is still a lot of outstanding issues so now we are starting on the PMD," he said, referring to alleged "possible military dimensions" of Iran's nuclear activities, mostly before 2003.

On Sunday, Iran and the IAEA agreed a new seven-step plan to increase transparency, including a pledge by Iran to provide "information and explanations for the Agency to assess Iran's stated need or application for the development of Exploding Bridge Wire detonators".

These detonators, known as EBW's, can have "non-nuclear applications", noted IAEA in a November 2011 report.

The 2011 report detailed information made available to the IAEA, much of it thought to be provided by Western and Zionist intelligence, about suspected "possible military dimensions" of Iran's program.

Iran denies it ever worked on nuclear weapons technology and says the evidence outlined by the IAEA is fabricated.