The U.N. nuclear watchdog declined to say whether Iran had met a deadline on Thursday for starting to address suspicions it may have carried out atomic weapons research, adding to signs of limited progress so far, Reuters reported.
Under a cooperation pact agreed between the two sides in November, Iran was to implement seven transparency steps by May 15 to help allay international concern about its disputed nuclear program, which the West fears may have military ends.
On the most sensitive of those - for Iran to provide information about detonators that can, among other things, be used to set off an atomic explosive device - diplomats have said the U.N. atomic agency was seeking further clarification.
How Iran responds to questions about so-called Explosive Bridge Wire detonators is seen as a litmus test of its readiness to begin cooperating with a long-stonewalled investigation into what the Vienna-based U.N. body calls the possible military dimensions (PMD) 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 U.N. International Atomic Energy Agency (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.
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 are holding a new round of negotiations this week, also in the Austrian capital.
U.S. officials say it is central 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.
According to an Iranian official, the U.S. delegation at the Vienna meeting said on Wednesday it wanted to discuss the PMD issue, but the Iranian side had ignored the request.
A U.S. official declined to comment, but referred to remarks earlier in the week by a senior U.S. official that "every issue" related to Iran's disputed atomic activities must be addressed in order for a final accord to be reached on curbing the country's nuclear program in exchange for an end to sanctions.
Nuclear experts David Albright and Bruno Tertrais, writing in the Wall Street Journal, said it was critical to know whether Iran "had a nuclear weapons program in the past, how far the work on warheads advanced and whether it continues".
"Without clear answers to these questions, outsiders will be unable to determine how fast the Iranian regime could construct either a crude nuclear-test device or a deliverable weapon if it chose to renege on an agreement."
NO "BIG DEAL"
The diplomatic sources said Iran in late April provided an explanation for what it said was a civilian application for the detonators but that the IAEA needed back-up documentation.
There was no indication that the issue was resolved at a meeting between Iran and the U.N. agency on Monday, after which the IAEA said only that Tehran had taken several actions but that "some related work continues".
In addition, the IAEA made clear that it and Iran had not yet agreed on what issues to tackle in the next phase under the cooperation pact. The U.N. agency wants to speed up its inquiry.
An IAEA spokeswoman said the U.N. agency was not planning to issue a statement on Thursday. Diplomats said a quarterly IAEA report on Iran's nuclear program, expected next week, will likely provide more information on the issue.
The other six measures that Iran agreed to take by Thursday's deadline - including inspector access to a uranium mine - were not seen as problematic. Iran says it has already implemented all seven steps.
"I assume the six other steps are not a big deal, so it is all about PMD," one Vienna-based envoy said.
The mere fact that Iran agreed to help clarify the detonator issue was seen as a breakthrough since the IAEA has tried for years, mostly in vain, to investigate allegations that Iran may have worked on designing a nuclear warhead.
It was, however, one of the least difficult issues that were detailed in a IAEA report in late 2011 that provided a trove of intelligence information pointing to past activities in Iran relevant to nuclear weapon development. Diplomats and experts say Iran must still do much more to address those points.