* Possible step forward in stalled IAEA probe
* West fears Tehran may seek nuclear weapons capability
* Iran says its programme peaceful; allegations false
* IAEA talks separate but complement big power diplomacy
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 the West says could help it build nuclear weapons.
The undertaking, agreed in secretive talks with the U.N. International Atomic Energy Agency in Tehran, could unblock the investigation the IAEA is trying to conduct and may also help Iran and six world powers negotiate a deal to end a nuclear dispute that raised fears of a new Middle East war.
In 2011, the IAEA issued a report that 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.
The IAEA said on Wednesday Iran would provide information about two issues in the report by Aug. 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 modelling and calculations and their alleged application to compressed materials".
The information Iran discloses on those issues will be seen by the IAEA as a test of its readiness to engage with the investigation into what the agency calls the possible military dimensions (PMD) of the its nuclear programme.
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 July 20 deadline.
But the Islamic state's repeated denials of any nuclear bomb aspirations will make it hard for it to admit to any illicit work in the past without a damaging loss of face.
IAEA SEES "GOOD PROGRESS"
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 programme as a cover for developing weapons capability.
"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," Ali Vaez, of the International Crisis Group think-tank, said.
But 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 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, agreed with the IAEA in November.
The other steps include a visit by the IAEA to an Iranian centrifuge research and development centre.
Although not directly related to the IAEA's investigation, such access is seen as important to give its inspectors greater insight into Iran's nuclear programme.
The IAEA said "good progress" had been made on seven measures that Iran had been due to implement by May 15, under a step-by-step cooperation pact aimed at helping to allay international concern about the country's nuclear programme.
But it did not spell out whether it was fully satisfied with Iran's explanations about the most sensitive of those steps - for Tehran to provide information about detonators that can be used, among other things, to set off an atomic explosive device.
Iran says it developed the detonators for civilian applications.