ISTANBUL (AFP) ― World powers Saturday failed to persuade Iran to take steps to ease suspicions over its nuclear program as the defiant Islamic republic insisted on uranium enrichment.

The two-day talks in Istanbul between Iran and the six world powers ended without progress and no new meeting was scheduled to tackle concerns that Tehran is secretly developing an atomic bomb.

“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,” EU foreign policy chief Catherine Ashton told reporters.

“No new talks have been planned,” she said.

Ashton headed the delegation of the P5+1 group, comprised of Britain, China, France, Russia, the United States and Germany.

In London, British Foreign Secretary William Hague said Iran’s “refusal to engage” was “extremely disappointing”.

French Foreign Minister Michele Alliot-Marie said it had been the conditions Iran had set ― including the lifting of sanctions ― that had scuppered the talks.

“This blocked everything,” she told reporters in Paris.

A senior U.S. diplomat said diplomatic channels nonetheless remain open.

“The door is open... We continue to believe that there is time and space for diplomacy,” he said on condition of anonymity.

The United States and Israel have not ruled out military action against Iran to prevent it from acquiring nuclear weapons.

Iran had set the stage for fierce wrangling as soon as the meeting began Friday, declaring that its sensitive uranium enrichment work was not up for debate.

Speaking shortly after Ashton, its chief negotiator Saeed Jalili insisted that Iran “has the right to a combustion cycle, including the enrichment of uranium,” under the nuclear Non-Proliferation Treaty.

“This right must be recognized,” he said, stressing that “we are ready for talks, even tomorrow” if the six powers were to accede to the long-standing Iranian demand.

Western sources familiar with the talks said the Iranians also insisted that international sanctions against Tehran be lifted as a precondition for talks on a possible nuclear fuel swap.

The Istanbul gathering was the second between the two sides after talks resumed last month in Geneva, breaking a 14-month hiatus in diplomatic efforts to settle the dispute.

Ashton stressed that while the Iranian “preconditions” blocked progress, the negotiating powers were committed to diplomacy.

The P5+1, she said, sought to negotiate a revised version of the nuclear fuel swap proposal and ways to improve transparency through monitoring measures by the U.N. atomic watchdog.

The powers believe the swap scheme, first discussed in 2009, could ease suspicions over Tehran’s activities, but say its terms should be modified in line with Iran’s growing stockpile of low-enriched uranium (LEU).

Under the original draft, Iran would have received fuel for a Tehran medical research reactor from France and Russia in return for shipping out 1,200 kilograms of LEU, or most of its stockpile at the time.

After a prolonged stalemate, Brazil and Turkey brokered a modified deal with Iran in May.

But the United States rejected that accord, arguing it had failed to take into account additional uranium Iran enriched in the meantime, and led the U.N. Security Council in imposing a fourth package of sanctions.

A viable modification of the terms would require a “great increase” in the quantity of LEU Iran should ship out, a Western official said in Istanbul.

The U.S. diplomat commented that Iran had tried and failed to split the P5+1 group, hoping to extract concessions,.

“If that was their calculation, they miscalculated because I have seen a 5+1 group more united as ever before,” he said.

Iran insists its nuclear program is peaceful, but has refused to suspend uranium enrichment, the sensitive process which can be used to make nuclear fuel or, in highly extended form, the fissile core of an atomic bomb.

Its defiance has prompted four sets of U.N. sanctions, coupled by a series of sanctions imposed unilaterally by the United States and the EU.