VIENNA A U.N. watchdog report on Iran's nuclear programme next week is expected to include evidence of research and other activities which make little sense if not weapons-related, Western diplomats said on Friday.
They said the keenly awaited document by the International Atomic Energy Agency (IAEA) will strengthen suspicions that Tehran is seeking to develop a capability to make atomic bombs but stop short of explicitly saying that it is doing so.
"There are bits of it which clearly can only be for clandestine nuclear purposes," one Western official said. "It is a compelling case."
Diplomats and experts said the Vienna-based U.N. agency would, in a technical annex to its next quarterly report on Iran, give plenty of new details and also add to areas of concern it has briefly mentioned previously.
Western powers believe Iran is secretly trying to develop nuclear weapons, but Tehran denies this, saying it is enriching uranium only to power reactors for electricity generation.
Any evidence of nuclear weapons activities would strengthen calls for further sanctions against Iran. French President Nicolas Sarkozy said on Friday stricter sanctions were the key to reining in Iran's nuclear programme.
China, a key customer for Iranian oil which has been less enthusiastic than Western countries for sanctions against Tehran, called on Iran to "show flexibility and sincerity and have earnest cooperation with the IAEA."
The IAEA has in a series of reports over the last year made clear its growing fears as a result of intelligence it has obtained pointing to experiments and other work in Iran linked to the design of a nuclear warhead for a missile.
The agency -- tasked with preventing the spread of nuclear arms -- said in May it had information related to the possible development of what analysts describe as an atomic bomb trigger, a key part of any nuclear weapons programme.
It called on Iran to provide clarifications regarding "experiments involving the explosive compression of uranium deuteride to produce a short burst of neutrons."
It did not give further details at the time, but may do so in its new report, due to be submitted to IAEA member states in the middle of next week ahead of a November 17-18 meeting of the agency's 35-nation governing board.
A neutron initiator is a device that floods the core of highly enriched uranium with subatomic particles to touch off the chain reaction of a nuclear explosion. Refined uranium can have both military and civilian purposes.
Mark Fitzpatrick, of the International Institute for Strategic Studies think tank, said the IAEA probably had more information about Iranian experiments using uranium deuteride.
"This is a very strong indication of work on nuclear weapons," Fitzpatrick said.
Mark Hibbs, of the Carnegie Endowment for International Peace, said every country in the world with a past or current nuclear weapons programme had carried out neutron research, which he said posed a major technical challenge.
"You've got nuclear material that you are trying to bring to a critical reaction in a hurry, in microseconds, and to do that you have to inject neutrons into the nuclear material to start a fission reaction," Hibbs said.
"It is very precise science. You have to really know what you are doing." Hibbs added that he was not aware of civilian applications for such experiments, but did not rule them out.
Other work in Iran the IAEA has expressed concern about include uranium metallurgy and explosives testing.
"The activities of Iran with, for example, high explosives and neutron physics experiments are alarming. They do exist, and the question now is what the purpose of those activities is?" former IAEA deputy director Olli Heinonen told Reuters.
Many experts believe Iran is still a few years away from having a nuclear arsenal and that it is unclear whether it will eventually decide to pursue such arms.
U.S.-based analyst Peter Crail of the Arms Control Association said the report was likely to show that Iran carried out research and development to be capable of building nuclear weapons in the future, "not that it is building them now."
Iran has repeatedly dismissed as forged and baseless allegations of military nuclear aims. The IAEA's information is widely believed to come from Western intelligence services, but also through its own sources.
One of the world's largest oil producers, Iran says it needs to enrich uranium to fuel a planned network of nuclear power stations, and also that nuclear weapons are banned by Islam.
But its history of concealing sensitive nuclear activity and its refusal to suspend work that also can also yield atomic bombs have already been punished by four rounds of U.N. sanctions, as well as separate U.S. and European punitive steps.
"We share the IAEA's concerns about the possible existence in Iran of undisclosed nuclear activities involving military organisations," said a senior Western official.
Iranian President Mahmoud Ahmadinejad said last month that those seeking atomic bombs were "politically and mentally retarded" as the era of such arms of mass destruction was over.
"The overall budget of our national atomic energy agency is $250 million (156 million pounds), and the whole budget is aimed at peaceful activities," Ahmadinejad told CNN, according to a transcript.
(Editing by Jon Hemming)