VIENNA — 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, but mostly they are used in weapons research and therefore Iran’s stated development of them “is a matter of concern.”

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

More than two years of talks between the agency and Iran on addressing these claims failed to make progress. Iran denies it ever worked on nuclear weapons technology and says the evidence outlined by the IAEA is fabricated.

But following the election of Hassan Rouhani as Iranian president last year, the two sides have been trying a different approach, agreeing last November on six steps which have now been completed.

Progress has also been made in parallel talks between Iran and world powers, with Iran agreeing in November in Geneva to freeze parts of its nuclear program for six months in return for minor sanctions relief.

Talks between Iran and the six powers — the United States, Russia, China, Britain, France and Germany — on a long-term, “comprehensive” accord are due to start in Vienna on February 18. They are expected to last for months.