Six world powers and Iran began talks on Tuesday in pursuit of a final settlement on Tehran's disputed nuclear program in the coming months 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 on Monday that talks between Tehran and six world powers "will not lead anywhere" - while also reiterating that he did not oppose the delicate diplomacy.

Hours later a senior US administration official also tamped down expectations, telling reporters in the Austrian capital that 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 a November 24 interim deal that, halting a decade-long slide towards outright conflict, 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.

The stakes are huge. 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 - expected to last two or three days - began on Tuesday at the UN complex in Vienna.

Western officials said the talks were aimed at agreeing on how the negotiations would proceed in coming months and what subjects would have to be addressed. "We are basically setting the table for the negotiations," the senior US official said.

"Nobody is expecting a final agreement in this round but we are hoping for progress ... the aim is to create a framework for future negotiations," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, told reporters.

Despite his public skepticism about chances for a lasting accord with the West, Khamenei made clear Tehran was committed to continuing the negotiations between Iran and Britain, China, France, Germany, Russia and the United States.

"What our officials started will continue. We will not renege. I have no opposition," he told a crowd in the northern city of Tabriz on Monday to chants of "Death to America" - a standard reflexive refrain since the 1979 Islamic Revolution.

During a decade of fitful dialogue with world powers, Iran has rejected allegations by Western countries that it is seeking a nuclear weapons capability. It says it is enriching uranium only for electricity generation and medical purposes.

Reuters