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Foreign powers negotiating with Tehran on its nuclear programme say they are "disappointed" after a round of talks ended without progress in Istanbul.

EU foreign policy chief Baroness Ashton, who led the international team, said Iran had come to the talks with pre-conditions.

While no date was set for new talks, she said the "door remained open".

Iran's chief negotiator said any deal should be based on Tehran's right to uranium enrichment.

The US and other Western powers say Iran is trying to develop a nuclear weapon, a charge Iran denies.

A US diplomat, speaking on condition of anonymity, said Washington believed diplomatic channels remained open to tackle concerns over Iran's nuclear programme.

"We continue to believe that there is time and space for diplomacy," the diplomat told AFP news agency.

Revised deal

Baroness Ashton led a delegation from France, Germany, Russia, the UK, US and China, at the two-day talks.

The international team had proposed a new version of a plan to keep Iranian enrichment within civilian-grade levels by having it send uranium abroad to be processed.

But Western diplomatic sources have spoken of anger and frustration over Iran's behaviour at the talks.

Baroness Ashton said Iran's pre-conditions were "not the way to proceed".

"We had hoped to embark on a discussion of practical ways forward, and have made every effort to make that happen. I am disappointed to say that this has not been possible," she said. "No new talks have been planned."

Baroness Ashton added: "The door remains open, the choice remains in Iran's hands."

She told reporters that her team had offered Iran "practical steps... to move forward".

One of the steps mentioned by Baroness Ashton was an updated fuel swap plan.

No details were given of how the plan, rejected in its original form by Iran in 2009, had been updated.

The original proposal, brokered by the UN's International Atomic Energy Agency (IAEA), would have seen Iran sending 70% of its low-enriched uranium (fuel enriched to 3.5%) to Russia for re-processing into higher-grade (20%) fuel rods.

The fuel rods would have then been returned to Iran for use in a research reactor.

The plan was designed to stop Iran having an excuse to further enrich uranium, or work towards enriching to the degree necessary to make a bomb (90%).

'Iran's right'

Iran's chief negotiator Saeed Jalili said after the talks ended: "Any kind of talks and co-operation, as I underlined during the talks with Mrs Ashton, should be based on respecting nations' rights, including Iran's right to nuclear technology."

He denied setting pre-conditions for the talks and accused the six powers of seeking to impose their own preconditions, pushing not "dialogue but dictation".

Iran had offered to swap batches of low-enriched uranium for fuel rods but on its own soil.

Correspondents say Iran feared it might not get back its uranium and end up with no fuel rods at all.

The scheme began to look redundant last February when Iran announced it was itself enriching uranium to 20%.

The UN has imposed four sets of sanctions on Iran over the years.

While these have make it more difficult for Iran to acquire equipment, technology and finance to support its nuclear activities, they have not stopped trading in oil and gas - the major sources of Iran's income.