By Fredrik Dahl

VIENNA (Reuters) - Iran may be holding back from working with a U.N. investigation into its nuclear program to use it as a bargaining chip in pursuit of significant sanctions relief or other concessions in broader negotiations with world powers.

That could explain why United Nations nuclear inspectors once again returned empty-handed after talks last week in Tehran, where they tried to overcome obstacles to a long-stalled inquiry into suspected atomic bomb research by Iran.

Iran has suggested at various times in the past that it would expect a "kind of reward" for cooperating with the International Atomic Energy Agency (IAEA), a Western official said, making clear he saw no rationale for this.

If this is Tehran's thinking, a year-long effort by the IAEA to unblock its investigation looks unlikely to succeed as long as separate diplomacy between the six major powers and Tehran remains deadlocked.

"They don't want to offer substantive cooperation," one Western diplomat said after the IAEA's latest stab on January 16-17 at coaxing the Islamic Republic into starting to address questions about its atomic activities.

Another envoy in Vienna, where the U.N. agency is based, described the IAEA's roller-coaster negotiations with Iran as a "well-practiced dance" by Tehran of "two steps backwards, one step forward."

The failure to achieve a breakthrough in the most recent of a series of largely fruitless meetings between the IAEA and Iran marked another setback for diplomatic efforts to resolve the stand-off and head off the threat of a new Middle East war.

Both Iran and the United States, Britain, France, Germany, China and Russia say they want to resume talks after a seven-month hiatus. But the two sides' priorities diverge: the powers want to curb Iranian nuclear work of potential use in developing atomic weapons, while Iran wants sanctions scrapped and their "rights" to enrich uranium formally recognized.

They have yet to announce a date and venue and as delays continue, Iran is amassing more nuclear material that could be turned into bomb fuel if refined further. Israel has threatened military action to foil any nuclear weapons capability in Iran.

The powers, known as the P5+1 as they group the five permanent, veto-wielding members of the U.N. Security Council along with Germany, want peaceful guarantees on Iran's enrichment program and Iranian transparency toward the IAEA.

Iran, which says it seeks only peaceful nuclear energy from enrichment, is keen above all for the West to remove sanctions expanded last year to block its economically vital oil exports.

"Tehran apparently is seeking to withhold cooperation with the IAEA in order to increase leverage vis-a-vis the P5+1," said Daryl Kimball of the Arms Control Association, a U.S.-based research and advocacy group.

NO REWARD

The IAEA, whose mandate is to forestall the spread of nuclear weapons, has been trying for a year to negotiate a framework agreement with Iran giving its inspectors access to sites, officials and documents for their investigation. Continued...