By Fredrik Dahl

VIENNA (Reuters) - The U.N. nuclear watchdog sought in talks with Iran on Tuesday to advance a long-stalled investigation into Tehran's atomic activities, but it was not immediately clear whether any headway was made.

A spokeswoman for the International Atomic Energy Agency confirmed the two sides met in Tehran, but said the IAEA was not planning to issue a statement about the talks on Tuesday, leaving open the possibility one might be made later.

Diplomatic sources had said the Vienna-based U.N. agency and Iran were expected to discuss IAEA requests for information about detonators that can, among other things, be used to set off a nuclear explosive device.

It could have provided an opportunity for progress on a key issue ahead of a quarterly IAEA report on Iran's nuclear program, due this week, which will be debated by the U.N. agency's 35-nation governing board at a meeting in early June.

Under a phased cooperation pact agreed between the two sides in November, Iran was to take seven transparency steps by May 15 to help allay international concern about its nuclear program, which the West fears may be part of a military project.

On the most sensitive of those - for Iran to provide information about the development of so-called Explosive Bridge Wire detonators - diplomats have said the U.N. atomic agency was seeking further clarifications from Tehran.

How Iran responds to the IAEA's questions is regarded as a litmus test of its readiness to start engaging with the investigation into what the U.N. agency calls the possible military dimensions of the country's nuclear program.

Iran denies Western allegations that it has been seeking to develop the capability to make nuclear weapons but has offered to work with the Vienna-based IAEA to resolve its concerns.

The IAEA-Iran talks are separate from those between Tehran and six world powers - the United States, France, Germany, Britain, China and Russia - aimed at reaching a broader deal to settle the decade-old nuclear dispute by late July.

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But they are complementary as both focus on fears that Iran may covertly be seeking the means and expertise to assemble nuclear weapons. Iran and the powers held a new round of negotiations last week, but made little progress.

U.S. officials say it is vital for Iran to resolve IAEA concerns for a successful outcome of the broader diplomacy. But Iranian denials of any atomic bomb aspirations will make it difficult for Tehran to admit to any illicit work in the past.

Diplomats said Iran in late April provided an explanation for what it said was a civilian application for the detonators but that the IAEA needed verification documents. A May 12 meeting in Vienna apparently failed to fully resolve the issue.

Also, the two sides have yet to announce any agreement on what issues to tackle in the next phase under last year's agreement. The IAEA wants to speed up its inquiry into suspected atomic bomb research by Iran.

(Editing by Alison Williams)