Iran "will not retreat an iota" from its nuclear rights, President Mahmoud Ahmadinejad said on Thursday, ahead of weekend talks in Istanbul with world powers over Tehran's atomic activities.

"The Iranian nation is standing firm on its fundamental rights and under the harshest pressure will not retreat an iota from its undeniable right," Ahmadinejad said in a speech in the southern town of Minab, according to the official IRNA news agency.

"On behalf of the Iranian nation, I advise the enemies and the arrogance (the United States) to change their behaviour towards our nation, and they should know that the Iranians are standing firm in defending their rights," he said.

His language indicated a defiant attitude by Iran as its negotiators go into the Istanbul talks on Saturday across from representatives of the so-called P5+1 group comprising the five permanent UN Security Council members plus Germany.

It will be the first negotiations since talks broke off in a failed round in January 2011 that was also held in Istanbul.

Tehran's stated aim in the upcoming talks is to have the world recognise its rights to peaceful nuclear energy -- including the contentious issue of enriching uranium, which, when purified to high levels, can be used to make an atomic bomb.

Iran denies it is seeking nuclear weapons.

But most of the West is unconvinced, and is piling on economic sanctions to get Tehran to agree to more invasive inspections and a cap on uranium enrichment to prevent any nuclear military activities.

If fully implemented, the sanctions could cut Iran's oil production by more than a third by the middle of 2012 should other customers not step forward, the International Energy Agency said in its monthly report released on Thursday.

"Iran's traditional crude buyers are struggling to arrange payment mechanisms, secure ships to lift the oil and insurance companies to underwrite the trade," the IEA noted.

The United States and its Middle East ally Israel have threatened military strikes against Iranian facilities if diplomacy on the nuclear issue fails.

But Iranian officials from the supreme leader, Ayatollah Ali Khamenei, on down have underlined that Iran will not respond positively in the talks if they are held under the pressure of sanctions or threats of military action.

"We have said that we do not have atomic weapons and we will not build any. But if there is any attack by the enemies, whether it be United States or the Zionist regime (Israel), we will attack them at the same level as they attack us," Khamenei said in his last speech, on March 20.

Both Ahmadinejad and Iran's chief nuclear negotiator, Saeed Jalili, on Wednesday warned the West to drop its "language of threat and pressure" and adopt a respect stance.

Iranian officials have also been focusing on an April 1 statement by US Secretary of State Hillary Clinton saying the P5+1 wants to verify declarations by Khamenei that Iran views nuclear weapons as forbidden under Islam.

This week, the head of Iran's judiciary, Ayatollah Sadeq Larijani, stressed that Khamenei's word was "the greatest possible guarantee" of Iran's peaceful nuclear intentions, according to IRNA.

Clinton said the US position in the talks was for Iran to end enriching uranium to 20 percent and ship out its existing stocks, and to open itself up to more intense inspections by the UN nuclear watchdog.

On Wednesday, she said: "We believe there is still time for diplomacy, but it is urgent that the Iranians come to the table to establish an environment conducive to achieving concrete results through a sustained process."