World powers will present Iran with an updated and "good" offer at talks this week on its nuclear programme, an EU official said Monday, although hopes for a breakthrough were slim.

Talks aimed at curbing Tehrans nuclear drive start in Kazakhstan Tuesday, with the so-called 5+1 world powers represented by the European Union sitting down with an Iranian team led by its top nuclear negotiator Saeed Jalili.

"We have prepared a good and updated offer for the talks, which we believe is balanced and a fair basis for constructive talks," said the spokesman for EU foreign policy chief Catherine Ashton.

"The offer addresses international concerns... on the nature of the Iranian nuclear programme, but is also responsive to Iranian ideas," said the spokesman, Michael Mann.

"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 the offer would still insist 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.

"This still forms the basis of the demands of the 5+1 group," said the source who asked not to be identified.

Another Western source said the powers could discuss lifting sanctions on Iran in exchange for specific concessions, although the source provided no further details.

Earlier reports said the power could consider easing sanctions on Iran's gold and precious metals trade.

World powers have until now wanted Iran to take the first step and only then reward it by easing the various sanctions regimes in place.

However Jalili said at the weekend that Tehran would not go beyond its obligations or accept anything outside its rights under the non-proliferation treaty (NPT).

"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 identified.

The 5+1 world powers are Britain, China, France, Germany, Russia and the United States.