The United Nations nuclear watchdog has received an explanation from Iran about the development of detonators that can help set off an atomic explosive device, and has asked for further clarification, diplomatic sources said on Friday, according to Reuters.

How Iran responds to the UN agency’s questions about so-called Exploding Bridge Wire (EBW) detonators is seen as an important test of its willingness to cooperate fully with a long-stalled probe into suspected atomic bomb research.

Iran says allegations of such activity are baseless, but has offered to help clear up the issues with the International Atomic Energy Agency (IAEA), according to Reuters.

Iran provided information to the IAEA about the fast-functioning detonators, which it says are for civilian use, in late April, the sources said.

They said the IAEA had asked Iran follow-up questions, but they did not give details of these and there was no immediate comment from the IAEA or Iran.

“Answering questions about EBW is significant – assuming the answers are substantive and sincere – because it gets to the heart of one of the sticky issues involving allegations of past nuclear work of a possible military dimension,” Mark Fitzpatrick, director of the non-proliferation program at the International Institute for Strategic Studies (IISS) think-tank, told Reuters.

Iran and the IAEA agreed in November on a step-by-step process to address allegations that Tehran may be seeking to develop a nuclear weapons capability.

As one of seven measures to be implemented by May 15, Iran agreed to provide information on the EBW detonators.

The mere fact that Iran agreed to address the issue was seen as a breakthrough as the IAEA has tried for years, mostly in vain, to investigate allegations that Iran may have worked on designing a nuclear warhead.

Iran and the six world powers, known as the P5+1, reached a six-month interim deal in November. Under the deal, which went into effect in January, Iran agreed to freeze its uranium enrichment program in return for sanctions relief worth some $6-7 billion, including the transfer of some $4.2 billion in frozen overseas funds.

Part of the deal includes regular inspections of Iran’s nuclear sites by IAEA inspectors. UN experts visited the heavy water plant at the unfinished Arak reactor on December 8, when all of the IAEA’s “technical objectives” were met, said the Vienna-based agency.

According to an IAEA report from March, Iran was sticking to a partial nuclear freeze as agreed upon in the interim deal.