By Fredrik Dahl VIENNA (Reuters) - The U.N. nuclear watchdog is concerned about Iran's current lack of engagement with an investigation into its suspected atomic bomb research, ahead of a deadline next month for Tehran to step up cooperation, diplomatic sources said on Tuesday.
Western officials want Iran to address questions by the International Atomic Energy Agency (IAEA) on allegations of past efforts to develop a nuclear weapons capability, something the country denies.
They say Iran clarifying the IAEA's concerns would also influence a diplomatic push by six world powers to negotiate an end to a decade-old standoff over the Islamic Republic's nuclear program, suggesting some sanctions relief may depend on it.
Iran says it is a peaceful project to generate electricity.
It rejects the IAEA's suspicions as based on false and fabricated information from its enemies, but has promised, since pragmatist Hassan Rouhani became president in mid-2013, to work with the Vienna-based U.N. agency to clear them up.
Under a phased cooperation pact hammered out late last year, an attempt to jumpstart the long-stalled IAEA investigation, Iran agreed two months ago to implement five nuclear transparency measures by Aug. 25, two of which directly dealt with the nuclear bomb inquiry.
However, so far there appears to have been little - if any - movement by Iran to engage on them, the sources said on condition of anonymity.
They said there was still time for Iran to meet its commitments, noting that it in the past occasionally had waited until the last minute, for example when it provided details in May about another issue that forms part of the IAEA's probe.
But the slow pace of cooperation may reinforce an impression in the West about continuing Iranian reluctance to give the IAEA the information and access to sites and people that it says it needs for its investigation.
There was no immediate comment from Iran or the IAEA.
U.S. officials say it is vital for Iran to address the IAEA's suspicions if the parallel negotiations between Tehran and the United States, France, Germany, Britain, China and Russia on a long-term deal to end the dispute are to succeed.
Those talks - which were extended by four months after the sides failed to meet a July 20 deadline for an accord - aim to set verifiable, civilian limits to Iran's nuclear program and lift punitive sanctions.
"PAST SINS"
The IAEA's inquiry focuses specifically on what it calls the possible military dimensions (PMD) of Iran's atomic activities.
After years of what the West saw as Iranian stonewalling, Iran as a first step in May gave the IAEA information it had requested as to the purpose in developing Exploding Bridge Wire detonators, which can be used to set off an atomic explosive device. Iran says it was for civilian use.
But, the diplomatic sources said, it does not appear to have started moving on the two PMD issues it agreed to clarify by late August - concerning alleged work on explosives and computer studies related to calculating nuclear explosive yields.
They were among 12 specific areas listed in an IAEA report issued in 2011 with a trove of intelligence indicating a concerted weapons program that was halted in 2003 - when Iran came under increased international pressure - but also suggesting some activities may later have resumed.
A U.S. official described the IAEA's investigation as one among "very difficult subjects" in the Iran nuclear diplomacy.
"That said, we have discussed a way forward on PMD, how we can help leverage these negotiations to get the kind of cooperation necessary to meet what the IAEA has set out," the official said, speaking on Saturday after Iran and the six powers agreed to extend their negotiations until Nov. 24.
Iran's denials of nuclear arms aspirations - saying such arms are banned by Islamic values - could make it virtually impossible for it to own up to any illicit work.
"Achieving an Iranian confession of past sins is not going to happen," Greg Thielmann, a senior fellow at the Arms Control Association research and advocacy group, said.
Instead, the focus should be on "strict limits on Iran's nuclear capability and intrusive" monitoring, he added.
(Editing by Raissa Kasolowsky)