Iran heads into nuclear talks with world powers this week in an apparently uncompromising mood, demanding Thursday the immediate recognition of its right to enrich uranium and downplaying the chances of direct talks with the United States.

The six nations negotiating with Iran have expressed cautious optimism as each side settled in for two days of meetings opening Friday in the Kazakh city of Almaty -- the fifth such round of negotiations on Iran's nuclear drive in two years.

Iran's chief nuclear negotiator pronounced the last get-together at the same venue in February "positive" but said the so-called P5+1 -- the five permanent UN Security Council members and Germany -- had to quickly agree to its most important demand in the decade-old dispute.

"We think that they can open up tomorrow's (Friday's) talks with one phrase -- and that is to accept Iran's right, particularly its right to enrich," Saeed Jalili said in a speech at an Almaty university ahead of the negotiations.

"We hope that in Almaty, they do not repeat the bitter experience they have gone through in the 34 years of our revolution and that they make the right conclusion this spring," he said referring to the 1979 Islamic revolution that ousted the shah.

The demand is inherently objectionable to the P5+1 because Iran is prohibited from enriching uranium by the United Nations and is heavily sanctioned for its secretive work.

UN Secretary General Ban Ki-moon said it would be on the Iranian side to prove that its "nuclear development program is for peaceful purposes".

"I sincerely hope that through this P5+1 negotiation there will be very meaningful progress," he told a press conference in Madrid.

The world powers are hoping for an answer from Tehran on a package of proposals aimed at defusing the standoff, offering an easing of punishing economic sanctions if it makes concessions on its enrichment.

Jalili also appeared to downplay the chances of a one-on-one meeting with chief US negotiator Wendy Sherman -- talks Washington has been seeking for years with its archfoe.

"What our nation is expecting is for the US to correct its behaviour, and not in just words, and tomorrow in Almaty they are in for another test," said Jalili.

"Those who come to negotiations should come with logic and not threats, saying that all the options are on the table. This is contrary to common sense."

The P5+1 is particularly concerned about Iran's enrichment to levels of up to 20 percent and wants it to shut the Fordo fortified bunker where the sensitive activity is conducted. The group also wants Iran to ship out its existing stockpile of 20-percent enriched material.

In return, Tehran has reportedly been offered the right to deal in some precious metals and perform small financial transactions now prohibited by international sanctions.

Tehran, which denies it is developing the atomic bomb and argues that it needs its nuclear programme for peaceful medical and energy needs, has already described this approach as unbalanced.

But a senior US administration official said the offer on the table now was "balanced and very fair."

"How far we get in Almaty 2 depends on what the Iranians come back with in terms of a response on the substance to our proposal," the US official said.

The official refused to speculate about what would happen should Iran not accept the proposal's terms or present its own unacceptable counter-offer.

"I would hope that we are not at any last chance," said the official.

"I think if we're not sure about how much we've gotten and whether we have gotten enough, we'll go back and consult with capitals before we reach any ultimate conclusion here. So I think we have time and space to consider what we hear."

US President Barack Obama said last month that Iran was just "over a year or so" away from making a nuclear weapon -- if the political decision to do so was ever reached.

EU foreign policy chief Catherine Ashton -- who is the lead P5+1 negotiator -- said she was going into the meeting with her usual "cautious optimism".