Iranian negotiators Thursday rejected proposals by six world powers to curb Tehran's nuclear program, and demanded answers to their own counteroffer meant to alleviate concerns about Iran's ability to build atomic weapons. The open channels between Iran and the six-nation bloc — the five permanent Security Council members plus Germany — are seen as the most hopeful chances of outreach between Washington and Tehran in years. They also could push back threats of military action that have shaken oil markets and brought worries of triggering a wider Middle East conflict. Saeed Jalili, Iran's top nuclear negotiator, demanded an overhaul to the plan put forward by the world powers after the Baghdad talks began Wednesday. An Iranian diplomat involved in the discussions said the package falls far short of a compromise. Jalili conveyed his concerns in a private meeting Thursday with the European Union's foreign policy chief Catherine Ashton, who is formally leading the talks. At the heart of the issue are two different proposals. On one side is an incentive package by the six-nation group — the United States, Russia, China, Britain, France and Germany — that seeks to halt the most sensitive part of Iran's nuclear fuel production. Iran, in turn, wants the US and Europe to ease harsh economic sanctions on its oil exports in return for pledges to give wider access for UN inspectors and other concessions. The West and allies fear Iran's nuclear program could eventually produce atomic weapons. Iran insists its reactors are only for energy and research. Iranian analyst Hassan Abedini called the proposal put forward by the US and its allies unbalanced and filled only with old plans that Tehran dismissed years ago. The Western package calls on Tehran to end its enrichment of uranium to 20 percent. That is the nuclear advance most worrying to the West since it hurdles technical obstacles to reaching 90 percent, or bomb-grade, enrichment. Iran says it will not exceed 20 percent and the material will be made into fuel for a research reactor. The powers also want Iran to send its stocks of higher-refined uranium abroad and close an underground plant devoted to enrichment and largely invulnerable to air strikes. Western diplomats ruled out any sanctions reprieve before Iran carried out its part of the deal in a verifiable manner, underlining the mutual mistrust accumulated over the course of a decade-long standoff. But Iranian media close to Tehran's delegation said it was insisting on a “principle of “reciprocity” of concessions they said was promised by the powers in preparatory talks in Istanbul last month but was not guiding the Baghdad negotiations. An Iranian delegate who asked not to be named poured cold water on hints from Western diplomats that the sides appeared to be nearing common ground on ideas for an outline deal. “What we heard in Istanbul was more interesting,” he said. “We believe the reason (the powers) are not able to reach a result is America. (They) came to Baghdad without a clear mandate so we think the atmosphere is difficult.” A UN nuclear agency report due in the next few days is expected to show that Iran has installed more uranium enrichment centrifuges at an underground site, potentially boosting output capacity of nuclear work global powers want it to stop.