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

(Reuters) - U.N. nuclear watchdog chief Mohamed ElBaradei began talks in Tehran on Friday to press for swifter Iranian cooperation with efforts to clarify its disputed atomic energy programme.

Following is an outline of issues about the programme still outstanding after four years of investigation, followed by issues clarified under a transparency plan agreed last August.

REMAINING QUESTIONS ABOUT PAST PROGRAMME

* The IAEA has U.S. intelligence indicating Iran has tried to "weaponise" nuclear materials by linking work on processing uranium ore, tests on high explosives and design of a missile warhead. After long rejecting the information as propaganda, Iran has begun substantive talks with IAEA experts on the matter. A subsequent U.S. intelligence report last month said Iran apparently stopped an active 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 -- bomb-grade -- uranium (HEU) found on equipment at physics research sites. The IAEA accepted Iranian explanations about other HEU traces found earlier that they came with equipment it had imported.

ISSUES ABOUT PRESENT PROGRAMME

* The IAEA wants Iran to observe its Additional Protocol, which permits short-notice inspections at locations beyond declared nuclear sites. The measure, which would allow access to workshops where Iran is developing an advanced enrichment centrifuge, is needed for the IAEA to verify there is no parallel activity geared to yielding bombs, rather than the goal of Iran's declared programme -- electricity. Without it, the IAEA cannot verify Iran's nuclear work is wholly peaceful.

* Iran has refused to suspend uranium enrichment despite resolutions by U.N. Security Council and IAEA Board of Governors demanding that it do so to defuse mistrust. Instead, it has rapidly expanded an underground centrifuge production plant.

* Last April Iran stopped giving the IAEA advance design data on planned nuclear sites in retaliation for sanctions. The IAEA has repeatedly urged Iran to reconsider the move.