Iran said Friday that its hopes of a breakthrough at long-awaited talks on its nuclear programme had taken a dive after warnings from the West that Tehran had to prove its credibility.

Following a 15-month hiatus, senior diplomats from the United States, Russia, China, Britain, France and Germany -- known as the P5+1 -- will gather at the negotiating table with Iranian counterparts in Istanbul on Saturday.

But the build-up has underlined the levels of mistrust and major differences to overcome, with a source close to Tehran's delegation saying that Western comments ahead of the talks did not "give us much hope."

"So far the Iranian delegation finds the Western position, as stated during the G8 meeting (on Thursday) and expressed in the media, disappointing and discouraging," the source, who wished to remain anonymous, told AFP.

Foreign ministers from the Group of Eight major industrialised nations called Thursday on Iran to begin a "constructive and serious dialogue" while highlighting Tehran's "persistent failure to comply with its obligations."

US Secretary of State Hillary Clinton also fired a warning shot, saying the Istanbul talks were "a chance for Iran to credibly address the concerns of the international community."

"In previous meetings Iran brought up political issues with no link to the nuclear issue and imposed unacceptable preconditions," one source within the P5+1 said in Istanbul.

"If Iran turns up for the meeting in the same spirit of 'Istanbul I', we're not going to get very far," the source told AFP, referring to the last stab at talks 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, dismantle its Fordo nuclear facility and send its enriched uranium stockpiles abroad.

Western powers fear that if Iran were to take the decision to develop the bomb, it could relatively quickly reconfigure Fordo's centrifuges to enrich to weapons-grade levels of 90 percent.

Comments from Iran indicate that Tehran is unlikely to bow to anything that infringes on its right to a peaceful nuclear programme for generating electricity and making isotopes for medical purposes.

President Mahmoud Ahmadinejad insisted this week that his country would not "retreat an iota from its undeniable right" to peaceful nuclear activities.

"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.

However, Russian Foreign Minister Sergei Lavrov was more upbeat, saying "there are certain hopes related to the meeting" given Iranian promises of "new initiatives" on the table in Istanbul.

The head of Russia's delegation, Deputy Foreign Minister Sergei Ryabkov, who held talks on Friday with the Iranian delegation -- who also met the Chinese -- warned against "overblowing the differences."

Saeed Jalili, who is heading the Iranian negotiating team and who was due to have dinner with the EU foreign policy chief Catherine Ashton late Friday, did not elaborate on what he might offer.

But the Iranians would likely get a "positive" reaction if they "offer something on 20 percent", a source in one of the European delegations told AFP on Friday.

Other diplomats said the immediate objective was modest -- finding enough common ground and signs of willingness to cooperate in order for talks to begin in earnest at another meeting in a few weeks, possibly in Baghdad.

"Iran's most recent response specifically said that they are prepared to sit down and talk about the nuclear issue. For us tomorrow is about testing that," another diplomat close to the talks said.

"We don't expect to get a lot of detail tomorrow 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 P5+1 are expected to press Iran to give the UN nuclear watchdog, the International Atomic Energy Agency, greater access to ease fears that it might have covert facilities, as Fordo was until late 2009.

They may also press Iran to answer accusations made in a damning IAEA report in November which took suspicions that Iran wants the atomic bomb to a new level and sparked speculation Israel might launch military action.

The IAEA report cited "overall, credible" evidence from different sources that at least until the end of 2003, and possibly since, Tehran carried out "activities... relevant to the development of a nuclear explosive device."

Since then Western countries have imposed increasingly severe economic sanctions on the Islamic republic, with an EU oil embargo and new US sanctions set to come into effect in July.