FACTBOX-Key issues in U.N. bid to curb Iran nuclear programme

(Reuters) - Iran has allowed top U.N. nuclear monitors to visit an advanced centrifuge research site for the first time in a gesture of transparency about its disputed atomic drive, diplomats said on Wednesday.

One of the diplomats, close to the International Atomic Energy Agency, said the IAEA was nearing the end of an inquiry into Iran's nuclear activity and cited concern a new big power move to increase sanctions on Tehran could hurt the process.

Following is an outline of issues remaining or resolved.

* OUTSTANDING QUESTIONS ABOUT PAST PROGRAMME:

-- The IAEA has U.S. intelligence indicating Iran has tried to weaponise nuclear materials by linking processing of uranium ore, tests on high explosives and design of a missile warhead.

-- After long rejecting the information as propaganda, Iran started substantive talks with IAEA experts on the matter, according to diplomats close to the agency.

-- A subsequent U.S. intelligence report last December said Iran apparently stopped an active, covert atom bomb programme in 2003. But Western diplomats remain sceptical of Iran's readiness to address weaponisation fully for fear of self-incrimination.

-- Inspectors want credible explanations for traces of highly-enriched -- or bomb-grade -- uranium (HEU) found at physics research sites, as well as the role of their scientists, what equipment they procured and why. The IAEA accepted Iranian explanations about other HEU traces found earlier that they came with equipment obtained from a Pakistani-led smuggling network.

* ISSUES ABOUT PRESENT PROGRAMME