BAGHDAD â€"  Iranian negotiators on 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 the Islamic Republic's ability to build atomic weapons.

The stance underscored the difficulties facing the nuclear talks as both sides stake out their terms and agendas for a second day in the Iraqi capital. Still, the negotiations did not appear in danger of collapse. Envoys added extra hours to their meetings as a sandstorm closed down the Baghdad airport.

Proposals for another round next month in Geneva also met with resistance from Iran, which is pushing for a venue not considered supportive of Western sanctions. Talks were expected to wrap up later Thursday.

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.

Israeli leaders have been critical of the talks, claiming it allows Iran to buy time and drive a wedge between Washington and Jerusalem.

On Wednesday, Israel's defense minister Ehud Barak said even possible moves by Iran to open its nuclear facilities to greater U.N. inspect doesn't rule out a possible Israeli military strike.

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.

Iran went into the talks seeking that the West scale back on its sanctions, which have targeted Iran's critical oil exports and have effectively blackballed the country from international banking networks.

Jalili conveyed his concerns in a private meeting Thursday with European Union foreign policy chief Catherine Ashton, who is formally leading the talks.

Ashton's spokesman, Mike Mann, called the negotiations "tough," but said that "some progress was made."

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 U.S. and Europe to ease harsh economic sanctions on its oil exports in return for pledges to give wider access to U.N. inspectors and other concessions.

The West and its allies fear Iran's nuclear program could eventually produce atomic weapons. Iran insists its reactors are only for energy and research.

A senior U.S. official predicted the pace of the talks - which began last month in Istanbul - would speed up in upcoming rounds.

"We are urgent about it, because every day we don't figure this out is a day they keep going forward with a nuclear program," said the U.S. official who spoke on condition of anonymity to discuss the negotiations more candidly. "And there are all kinds of assessments about how long it will take them to get there."

"We still think we have some time for diplomacy, but it's not indefinite," the official said.

Iranian analyst Hassan Abedini called the proposal put forward by the U.S. and its allies unbalanced and filled only with old plans that Tehran dismissed years ago.

jIn exchange, the world powers offered benefits, including medical isotopes, some nuclear safety cooperation and spare parts for civilian airliners that are needed in Iran.

But they snubbed Iranian calls for an immediate easing of significant economic sanctions imposed on Tehran for flouting U.N. Security Council resolutions that demand the suspension of all enrichment.

"Giving up 20 percent enrichment levels in return for plane spare parts is a joke," said Abedini. "The package is unbalanced and therefore unacceptable."