FACTBOX: Key issues in U.N. bid to curb Iran nuclear program

(Reuters) - The International Atomic Energy Agency said on Sunday that Iran has agreed to answer lingering questions about its past secret nuclear work within a month during high-level talks in Tehran.

U.N. nuclear watchdog chief Mohamed ElBaradei met top Iranian leaders last week to push for swifter cooperation with efforts to clarify its disputed atomic energy program.

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

* REMAINING QUESTIONS ABOUT PAST PROGRAM:

-- The IAEA has U.S. intelligence indicating Iran has tried to "weaponize" 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 a bomb program in 2003. But Western diplomats remain skeptical of Iran's readiness to address weaponization 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 PROGRAM