VIENNA - Iran ignored a U.N. deadline on Friday to respond to an international draft deal for it to cut an atomic stockpile the West fears could be used for weapons, and challenged the basis of the pact.

Iranian officials said they would give an answer only next week to the U.N.-drafted deal, which has been accepted by the other parties -- Russia, France and the United States.

They also said Tehran preferred to acquire enriched uranium abroad rather than send out its own for processing into fuel for nuclear medicine, as Western powers said it tentatively agreed to at Geneva talks on October 1 on ways to defuse growing confrontation over its disputed atomic aspirations.

Their remarks suggested that instead of engaging with the IAEA's draft, Iran was following a well-tested strategy of buying time to blunt Western pressure for harsher international sanctions while it presses on with nuclear research.

The U.N. nuclear agency said it had been told by Iran that it was considering the proposal in depth and in a favorable light, but needed until the middle of next week to take a position -- flouting the IAEA's Friday deadline for responses.

It said International Atomic Energy Agency Director General Mohamed ElBaradei hoped Iran's reply will equally be positive, since approval of this agreement will signal a new era of cooperation after seven years of standoff.

The IAEA did not say why Iran required more time to decide.

It would require the Islamic Republic, whose nuclear secrecy and restrictions on IAEA inspections have raised alarm, to send 1.2 tons of its known 1.5-tonne stockpile of low-enriched uranium (LEU) to Russia and France by the end of the year.

There it would be further processed, in a way that would make it hard to use for warheads, and returned to Iran as fuel plates to power a Tehran reactor that makes radioactive medical isotopes but is due to run out of its imported fuel in a year.

The deal would test Iran's stated intention to use enriched uranium only for peaceful energy.

It would also gain time for broader talks on world powers' ultimate goal: that Iran allay fears that it has a secret nuclear weapons program by curbing enrichment, in return for trade and technology benefits.

But the stance taken by Iranian officials could call into question plans to resume talks at the end of October and offered little to douse fears of a nuclear breakout risk in Iran.

INSPECTORS HEADING TO HIDDEN ENRICHMENT SITE

Underscoring concerns, senior IAEA inspectors prepared to head for Iran to examine an enrichment site on Sunday revealed by Tehran last month after Western spy services penetrated a three-year veil of secrecy. They were expected to stay 2-3 days.

Buying enriched uranium abroad would not only fail to reduce the domestic stockpile worrying the international community, but also require sanctions imposed on Iran since 2006 to be waived to allow it to import such sensitive nuclear material.

Iran is interested in buying fuel for the Tehran research reactor within the framework of a clear proposal, Iranian state television quoted a member of Iran's negotiating team, who attended nuclear talks in Vienna this week, as saying.

We are waiting for the other party's constructive and trust-building response.

A U.S. State Department spokesman Ian Kelly said Washington still hoped Iran would answer positively next week. Obviously we would have preferred to have a response today. We approach this with a sense of urgency, he told a regular news briefing.

French Foreign Minister Bernard Kouchner was quoted as saying in Lebanon that via the indications we are receiving, matters are not very positive and said this augured ill for further talks between Iran and six big powers planned shortly.

If these indications remain negative and there is no consensus on the expert level ... this will reflect negatively on the continuation of the political contacts at the level of the 5+1 meeting in Geneva.

A senior developing nation diplomat in Vienna with good contacts with the Iranians said he doubted Iran would agree to transfer the bulk of its LEU stockpile abroad.

They will not want to lose much of their main bargaining chip, with negotiations pending on broader strategic issues in the nuclear file, he told Reuters.

ElBaradei hammered out the draft in three days of difficult consultations in Vienna with the four nations' delegations.

The deal would reduce Iran's reserve of LEU below the threshold that could yield enough fissile material for one nuclear weapon if refined to 90 percent purity. LEU is refined to 5 percent. The reactor's fuel is 20 percent pure.

The Islamic Republic says its nuclear energy program is only for producing electricity, but it is years away from having any nuclear power plants that would use LEU.

Iran has repeatedly rejected U.N. and IAEA calls on it to curb enrichment or grant unfettered U.N. inspections, meant to verify that it is not trying to develop nuclear arms covertly.

The six powers will pursue these issues with Iran at further talks at senior foreign ministry level set for Geneva. Iran's current pace of enrichment would replace the amount of LEU earmarked for processing abroad in a year or less.

(Additional reporting by Parisa Hafezi in Tehran and Sylvia Westall in Vienna; editing by Philippa Fletcher)

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