Iran's Foreign Minister Mohammad Javad Zarif said talks which kick off in Geneva on Thursday with world powers about his country's disputed nuclear programme will be "very difficult".

The West and Israel suspect Iran's nuclear drive may be aimed at developing atomic weapons, but the Islamic republic insists the programme is only for the generation of electricity and medical purposes.

"My colleagues and delegations from the P5+1 countries are starting very difficult negotiations because we have entered a detailed phase that is still difficult and precise," Zarif said on his Facebook page.

Zarif said he would have a breakfast meeting on Thursday with Catherine Ashton, the EU diplomatic chief who heads the so-called P5+1 group -- Britain, China, France, Russia and the United States plus Germany.

He would also meet in Geneva with Lakhdar Brahimi, the UN-Arab League peace envoy to Syria, before travelling to Rome for a "few hours" to meet his Italian counterpart Emma Bonino.

In their second meeting in the Swiss city in less than a month, negotiators will sit down with Iran for two days in the hopes of hammering out a framework agreement to end the long-running standoff.

"I believe it is even possible to reach that agreement this week," Zarif told France 24 television on Tuesday. "But I can only talk for our side, I cannot talk for the other side."

But the Western side has been more reserved, with Ashton's spokesman Michael Mann saying that the talks "are complex and have entered a serious phase".

Lead US negotiator Wendy Sherman has expressed hopes that the talks will be a "first step" to resolving the issue, and said that Washington is prepared to offer Iran "very limited, temporary, reversible" relief from sanctions.

Iran's senior negotiator, Deputy Foreign Minister Abbas Araghchi, has said that the fresh round of negotiations were "a test of the political will of the P5+1 to reach a solution" to end the nuclear crisis.