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

Following a 15-month hiatus, officials from the United States, Russia, China, Britain, France and Germany (known as the P5+1) will gather around 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, wishing to remain anonymous, told AFP.

Foreign ministers from the Group of Eight major industrialised nations called on 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."

Washington echoed the same sentiment on Friday, with US deputy national security advisor Ben Rhodes saying the US wanted a "positive environment" and for Tehran to show "seriousness."

US media reports meanwhile have suggested that the P5+1 could insist that Iran halt enrichment of uranium to purities of 20 percent, dismantle its Fordo nuclear facility and send its enriched uranium stockpiles to another country.

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.

"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.

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 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 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, warned as well against "overblowing the differences."

Saeed Jalili, who is heading the Iranian negotiating team, did not elaborate on the details of 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.

"For this round of talks in Istanbul, we are looking for an environment that is conducive to a sustained process that will lead to concrete progress," one Western official told AFP on condition of anonymity.

Amid a flurry of last-minute diplomacy Jalili and his delegation held meetings with the Russian and Chinese delegations and he was due to have dinner with the EU foreign policy chief Catherine Ashton, her spokesman told AFP.

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.

This includes more oversight at Fordo -- secret until late 2009 -- and permission to see sites such as the Parchin military base mentioned in a damning IAEA report in November which took suspicions to a new level that Iran wants the bomb.

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 to pressure it to halt its activities, notably uranium enrichment, amid speculation of pending Israeli military action.

Iran's top national security official Saeed Jalili (R) meets with Turkish Foreign Minister Ahmet Davutoglu in Istanbul. Iran said Friday that its hopes of a breakthrough at long-awaited talks on its nuclear programme have taken a dive after warnings from the West that Tehran had to prove its credibility. (AFP Photo/Bulent Kilic)