advancing a new offer but Tehran has already doused expectations by installing new centrifuges and saying it will not go beyond existing obligations.
The two-day meeting under the shadow of the Tien Shan mountains in the Kazakh city of Almaty comes as sanctions bite against the Islamic republic and Israel still refuses to rule out air strikes to knock out Iran's suspected nuclear weapons drive.
Little apparent progress has been made since the last such session of talks in Moscow in June 2012 ended without any breakthrough and the crux of the dispute remains Iran's insistence on not abandoning uranium enrichment operations.
"We don't expect any breakthrough. The Iranians have made different declarations in the last days. It depends if you take the positive or the negative ones," said one Western official who asked not to be named.
World powers will present Iran with a "good" offer, said Michael Mann, the spokesman for EU foreign policy chief Catherine Ashton.
"We hope that Iran will seize this opportunity and come to the talks with flexibility and commitment to make concrete progress towards a confidence-building step."
A source close to the negotiations said that the world powers in their offer would stick to their insistence that Iran halts enriching uranium to 20 percent, shuts down its controversial Fordo uranium enrichment plant and sends abroad all uranium already enriched to 20 percent.
Previous reports have said that Iran could in return be offered a softening of the sanctions regime against Tehran, possibly starting with a lifting of measures against its gold industry.
The atmosphere has already been clouded by a UN nuclear watchdog report saying Iran started installing next-generation centrifuges at its Natanz nuclear plant, a move Washington said would be "provocative".
Meanwhile, chief Iranian nuclear negotiator Saeed Jalili said Saturday Tehran will not go beyond its obligations or accept anything outside its rights under the non-proliferation treaty (NPT).
Iran's supreme leader Ayatollah Ali Khamenei, who has the last say in all foreign policy matters, has effectively rejected the offer of direct US talks and appeared to order a tough line in Almaty.
Khamenei defiantly claimed earlier this month that even though Iran has no intention of developing nuclear weapons, the United States could not thwart Tehran if it wanted to.
The talks involve the so-called 5+1 world powers on one side -- Britain, China, France, Germany, Russia and the United States -- and Iran's team led by Jalili on the other.
They come with the lingering threat of Israel launching a unilateral strike on Iran just as it had done against the Osirak nuclear reactor in Saddam Hussein's Iraq in 1981.
Such action would almost certainly drag the United States into a conflict it clearly wants to avoid and leave the global economy in peril due to the impact on the price of oil.
Strikes would also risk sparking a broader Middle East conflict -- a danger the region can hardly afford with the violence raging in Syria.
Analysts said Israel's "red line" would be a decision by Iran to enrich uranium above its current upper limit of 20 percent -- within reach of weapons-grade uranium but necessary for Iran's medical research.
Iran already has a nuclear power plant in the southern city of Bushehr -- built with Russian help -- but Khamenei has described atomic weapons as a "sin".