The first round of high-level negotiations on a long-term agreement on Iran's nuclear programme have begun in Vienna.

The talks between six world powers and Iran come despite caveats from both sides that a breakthrough deal may prove impossible.

Iranian Supreme Leader Ayatollah Ali Khamenei, the man with the final say on all matters of state in the Islamic Republic, declared again yesterday that the talks "will not lead anywhere".

He also reiterated that he did not oppose the delicate diplomacy.

Hours later a senior United States administration official also lowered expectations.

The official said it will be a "complicated, difficult and lengthy process" and "probably as likely that we won't get an agreement as it is that we will".

It is the first round of high-level negotiations since an interim deal on 24 November that has seen Tehran curb some nuclear activities for six months in return for limited relief from sanctions to allow time for a long-term agreement to be hammered out.

If successful, the negotiations could help defuse many years of hostility between Iran and the West, ease the danger of a new war in the Middle East, transform power relationships in the region and open up vast new possibilities for Western businesses.

The talks, which are expected to last two or three days, began this morning at the United Nations complex in Vienna.

A spokesman for European Union foreign policy chief Catherine Ashton, overseeing the talks on the powers' behalf, said bilateral meetings between delegations were under way.

The US and its Western allies suspect Iran of trying to develop atomic weapons, a charge Tehran has steadfastly denied, insisting that its nuclear programme is peaceful.

The deal reached in Geneva last November angered Islamic hardliners, who accused the government of selling out to the west by making concessions on a matter of national pride.

Tehran and Washington have not had official ties since after the 1979 Islamic revolution that toppled the US-backed Shah, and Ayatollah Khamenei has vetoed any initiative for rapprochement.