"They called on Iran to seize the opportunity of the resumption of talks with the Six (world powers) to engage in serious negotiations and suspend all sensitive nuclear activities," Sarkozy's office said in a statement after the two leaders held video-conference talks.

"The two presidents noted their determination to apply sanctions with the greatest firmness as long as Tehran refuses to meet its international obligations, in particular Security Council resolutions on its military nuclear programme," the statement said.

World powers and Iran meet Saturday in Istanbul to discuss Tehran's promised "new initiatives" on its nuclear activities, after a 15-month break in talks.

The so-called P5+1 powers -- permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany -- suspect Iran is concealing its real purpose of producing atomic weapons.

The powers want Iran to stop enriching uranium to 20 percent, ship out its existing stocks and open itself up to more intense inspections by the UN nuclear watchdog.

The last round of talks between Iran and the world powers was held in Istanbul in January 2011, but failed to produce results.

Clinton urges Iran to 'credibly' answer nuclear concerns
Washington (AFP) April 12, 2012 - US Secretary of State Hillary Clinton on Thursday urged Iran to "credibly" answer international concerns over its suspect nuclear program when it meets world powers in Istanbul at the weekend.

"We continued to underscore that we hoped these talks result in an environment that is conducive to a sustained process that delivers results," Clinton told reporters after a meeting of Group of Eight foreign ministers.

"This is a chance for Iran to credibly address the concerns of the international community," she added.

"Iran, in coming to the table, needs to demonstrate that they are serious," the chief US diplomat said.

Iran is to meet Saturday in Istanbul with the United States, Russia, China, Britain, France and Germany to discuss its nuclear program, the first such talks since a similar meeting broke up inconclusively more than a year ago.

Most of those countries are represented in the Group of Eight which is composed of the United States, Canada, Russia, Britain, France, Germany, Italy and Japan.

The West suspects Iran's uranium enrichment program masks a drive to produce a nuclear weapon. Iran denies the charge, insisting its program is purely peaceful.

"The Iranian delegation will have new initiatives and we hope that the other party will have a constructive approach," Iran's top nuclear negotiator said Wednesday, raising hopes that Tehran might have a plan to change dynamics.

What Iran will bring to the table Saturday remained unclear after Saeed Jalili's words, but the very fact that the talks are starting again after a 15-month break is seen as a crucial opportunity to lower the tension.

But a senior European diplomat warned against over-optimism, pointing to a lack of "positive signals" from Tehran that compromises will be made to finally lift Western fears surrounding Iran's nuclear activities.

The so-called P5+1 powers -- permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany -- suspect Iran is concealing its real purpose of producing atomic weapons.

Tehran denies any such intention, saying it only wants to produce electricity.

The unlikelihood of compromise was underlined by Iranian President Mahmoud Ahmadinejad Thursday, as he ruled out the possibility of a climbdown.

"The Iranian nation is standing firm on its fundamental rights and under the harshest pressure will not retreat an iota from its undeniable right," he said Ahmadinejad.

Western countries have imposed increasingly severe economic sanctions on the Islamic republic to pressure it to halt its activities, notably uranium enrichment.

Iran instead has sought to accelerate its nuclear pursuit and hit back against its accusers.

Tehran announced on Wednesday that it was halting oil exports to European Union nations, as pre-emptive punishment of the EU for imposing an oil embargo on Iran that is to come into full effect on July 1.

For Iran's Foreign Minister Ali Akbar Salehi, the aim of Saturday's talks is to ensure that "Iran gains its rights and the P5+1 has its stated concerns alleviated."

Those rights include access to peaceful nuclear energy for Tehran, which sees uranium enrichment as a "right" that cannot be taken away and wants to keep its military sites off-limits to atomic energy inspections.

Jalili said that more pressure "only reinforces the determination of the Iranian people," but the European diplomat retorted that only this pressure had brought Iran back to the negotiating table.

"The fact is that every time we take another step in sanctions against Iran, they suddenly wake up and say they are ready to discuss," he told AFP on condition of anonymity. "There has got to be a link between the two."

The Western powers want Iran to stop enriching uranium to 20 percent, ship out its existing stocks and open itself up to more intense inspections by the UN nuclear watchdog.

Earlier this week the United States said it had no problem with Iran's production of peaceful, civilian nuclear power -- as long as it can satisfy the world that it does not intend to make weapons.

In response, Tehran says the target should be a nuclear weapons-free Middle East -- a clear reference to the undeclared arsenal of its arch-enemy Israel, which it says should be subject to the same inspections.

Israel, which sees a possible Iranian nuclear weapon as a threat to its existence, claims Iran may be on the cusp of "break-out" capability -- when it could quickly build a bomb -- and does not rule out a pre-emptive strike of its own.