Iran and world powers gathered Saturday for their first meeting in 15 months hoping to ease tensions over Tehran's nuclear programme, but with tough talk on both sides making major progress look unlikely.

According to officials close to the negotiations between Iran and the United States, Russia, China, Britain, France and Germany (the P5+1), the most that can be hoped for is an agreement to hold more detailed discussions in a few weeks.

"Iran's most recent response specifically said that they are prepared to sit down and talk about the nuclear issue. For us (Saturday) is about testing that," one envoy said ahead ahead of the talks from 0800 GMT between top diplomats.

"We don't expect to get a lot of detail ... but it will be about possibly meeting again in four to six weeks time if we can, when we will get into that detail," the diplomat said on condition of anonymity.

The main concern of the international community is Iran's growing capacity to enrich uranium, which can be used in power generation and other peaceful uses but, when purified further, for a nuclear weapon.

Of particular worry is the formerly secret Fordo site in a mountain bunker near the holy city of Qom, currently enriching to 20-percent purity but which experts say could be reconfigured to produce 90-percent weapons grade material.

Fordo's expansion, plus a major UN atomic agency report in November on alleged "weaponisation" efforts, have led to tighter EU and US sanctions on Iran's oil sector due to bite this summer and talk of Israeli military strikes.

Highlighting the deep mistrust and the major differences to overcome, a source close to the Iranian delegation led by Saeed Jalili said Friday that Western comments ahead of the Istanbul talks did not "give us much hope."

"So far the Iranian delegation finds the Western position ... disappointing and discouraging," the source, wishing to remain anonymous, told AFP.

Following earlier false starts, Western governments -- and in particular US President Barack Obama as he seeks re-election this November -- are wary of finding themselves accused of being duped by empty Iranian promises.

Obama, who has warned against "loose talk" of war, said last month that "both the Iranian and the Israeli governments recognise that when the United States says it is unacceptable for Iran to have a nuclear weapon, we mean what we say."

"I think that the Israeli government recognises that, as president of the United States, I don't bluff."

As a result the West has stressed the need for the Iranian "seriousness", with Group of Eight (G8) foreign ministers this week highlighting Tehran's "persistent failure to comply with its obligations."

"If Iran turns up for the meeting in the same spirit of 'Istanbul I', we're not going to get very far," one P5+1 source told AFP, referring to the last stab at talks in this Turkish city in January 2011.

US media reports meanwhile have suggested that the P5+1 want Iran to halt enrichment of uranium to purities of 20 percent, shutter Fordo and send Tehran's stockpiles of enriched uranium abroad.

Iran meanwhile has promised to present "new initiatives", but diplomats said it was unclear what these were or whether they would be enough to convince the P5+1 that further negotiations, possibly in Baghdad in May, would be worth it.

Turkish Foreign Minister Ahmet Davutoglu sounded upbeat after meeting Jalili late Friday, saying: "We will have good news at the end of the meeting."

The world powers also want Iran to grant the International Atomic Energy Agency greater access to ease fears that it might have covert facilities, and to answer accusations made in the IAEA's November report.

But comments from Iran, most notably from President Mahmoud Ahmadinejad, underline that Tehran will not consent to anything that infringes on its right to peaceful nuclear activities without promises that sanctions will be lifted.

"Iran will ultimately insist upon a guarantee ... that it has the right to enrich uranium," Mark Hibbs, analyst at the Carnegie Endowment for International Peace, told AFP ahead of the talks.

Iran's hardline press sounded a defiant note on Saturday, with the Kayhan daily saying Iran's "minimum expectation from the P5+1 is that they seek Iran's trust by cancelling all illegal resolutions and sanctions as the first step."