"It is necessary to show some voluntary flexibility that does not violate principles and red lines if the issue is pursued on an acceptable path," Ali Hosseini-Tash, a member of the Supreme National Security Council, told the student ISNA news agency.

Iran has been offered a package of incentives by the five permanent UN Security Council members if it agrees to suspend sensitive uranium enrichment -- a process in the nuclear fuel cycle that can also make the core of an atom bomb.

But Tehran considers enrichment to be its non-negotiable red line, and Hosseini-Tash reiterated that "suspension of peaceful nuclear activities is not a pre-condition" to talks over the incentive package.

The enrichment activities are the focus of concerns in the West that Tehran could acquire nuclear weapons, although Iran insists the program is only aimed at generating electricity.

Iran's top nuclear negotiator Ali Larijani and the European Union's foreign policy commissioner, Javier Solana, are to meet in Brussels on Wednesday on the package.

"The two sides will talk about their views on removing ambiguities and reaching a result," Iranian foreign ministry spokesman Hamid Reza Asefi said.

But the United States said Friday it expected Iran to respond to the international offer -- which it received June 6 -- at the Brussels meeting.

"We've seen lots of political statements from lots of political figures. We are waiting for the authoritative channel which is the Larijani channel to Solana," US Under Secretary for Political Affairs Nicholas Burns said in Brussels.

Iranian President Mahmoud Ahmadinejad had set August 22 as the date for Tehran's response.

Iran on Sunday again rejected a deadline to respond, with Asefi insisting that Iranian authorities were not trying to buy time.

"It is not a question of tactics and wasting time. It is a multi-dimensional package and takes time to examine," he said.

"There are ambiguities (in the package) which need to be discussed with the Europeans.

"We will submit a logical response considering our country's rights and interests," he said, adding that "alleviating the West's concerns should not be interpreted as sacrificing our interests."

Senior military officers have meanwhile warned the US administration that bombing raids against Iran would likely fail to destroy the country's nuclear program because of a lack of reliable intelligence, the New Yorker magazine reported Sunday.

Pentagon officers "have told the administration that the bombing campaign will probably not succeed in destroying Iran's nuclear program", Pulitzer Prize winning journalist Seymour Hersh wrote in the magazine's latest edition.

The officers are concerned about contingency plans to launch air strikes against Iran in the absence of reliable intelligence or concrete evidence of bomb making, the magazine said, citing unnamed active duty and retired officers and officials.

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