VIENNA, - Iran may be holding back from working with a United Nations 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 UN 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.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 UN 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 US-based research and advocacy group.After the previous meeting in mid-December, the IAEA said progress had been made and that it expected to seal the deal in this month's session. But after the two days of talks last week it said "important differences" remained.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 Jan. 16-17 at coaxing the Islamic Republic into starting to address questions about its atomic activities. Another envoy in Vienna, where the UN 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.