VIENNA (Reuters) - A long-stalled U.N. probe into suspected atomic bomb research by Iran took a potentially important step forward this week when Tehran agreed to address questions about explosives and other activity that the West says could help it build nuclear weapons.
The undertaking, hammered out in secretive talks in Tehran, could advance an investigation that the U.N. International Atomic Energy Agency is trying to conduct, and may also help Iran and six world powers to negotiate a broader deal to end a dispute that has raised fears of a new Middle East war.
But Western capitals, aware of past failures to get Iran to cooperate with the IAEA, are likely to remain skeptical until it
has fully implemented the agreed steps and others to clear up allegations of illicit atomic work.
An IAEA report in 2011 included intelligence information pointing to past tests and experiments in Iran that could be relevant for the development of nuclear weapons, something Iran denies it has ever sought. Tehran says the allegations are false but has offered to help resolve them.
The IAEA said on Wednesday that Iran would provide information about two issues covered in the report by August 25, including "with respect to the allegations related to the initiation of high explosives, including the conduct of large-scale high-explosives experimentation in Iran".
Iran would also give the U.N. agency explanations "related to studies made and/or papers published in Iran in relation to neutron transport and associated modeling and calculations and their alleged application to compressed materials". Computer calculations can be used to determine the yield of a nuclear explosion.
The information Iran discloses will be seen by the IAEA as a test of its readiness to engage with the investigation into what the IAEA calls the possible military dimensions (PMD) of its nuclear program.
U.S. officials say it is vital for Iran to address the IAEA's concerns if Washington and five other powers are to reach a long-term nuclear accord with Iran by a self-imposed deadline of July 20. But the Islamic state's repeated denials of any nuclear bomb aspirations will make it hard for it to admit to any wrongdoing in the past without losing face.
The IAEA-Iran talks are separate from those between Tehran and the United States, France, Germany, Britain, China and Russia. But they are complementary as both focus on fears that Iran may covertly be using a nuclear power and research program as a cover for developing a weapons capability.
After years of increasing hostility with the West, last year's election of the pragmatist Hassan Rouhani as Iranian president paved the way for an interim accord in November to curb Iran's nuclear program in exchange for some easing of sanctions, designed to buy time for talks on a final accord.
But the latest round of negotiations between Iran and the six powers - which want Tehran to significantly scale back its nuclear work - failed to make much headway last week, raising doubts over the prospects for a breakthrough by late July.
"The fact that there is progress in the Iran-IAEA talks is testament to Tehran's understanding of the critical importance of resolving the PMD issues for ending the nuclear crisis," said Ali Vaez, of the International Crisis Group think tank.
But he said the slow pace of progress indicated that the talks with the big powers were setting the tone.
A Western diplomat who is not from one of the six powers negotiating with Iran said he had expected more.
"The Iranians have said they want to get through these issues (the IAEA's probe) quickly. They will really have to pick up the pace or it will drag out a long time," the envoy said.
The two PMD issues that Iran has now agreed to address were among a package of five practical measures to be implemented by late August.
The IAEA said "good progress" had been made on seven other measures that Iran had been due to implement by May 15.
But it did not spell out whether it was fully satisfied with the most sensitive of those steps - an Iranian explanation for having detonators that can be used, among other things, to set off a nuclear explosive device.
Iran says it developed the detonators for civilian applications.
Editing by Robin Pomeroy and Kevin Liffey