A Western diplomat said the United States was "willing to sit at the table with the Iranians" along with the four other permanent UN Security Council members plus Germany to discuss a compromise package on the nuclear dispute.

The package, drafted by a European Union negotiating troika and due to be discussed at a meeting of world powers here Thursday, offers Tehran incentives in return for guarantees that it will not develop nuclear weapons.

But the diplomat said Washington would only join the multi-party talks "if Russia and China can agree on Thursday to key aspects of the package, including some specific future sanctions if Iran rejects it."

His comments were confirmed by another diplomat from a second member of the UN permanent five, which is made up of Britain, China, France, Russia and the United States.

They asked not to be named due to the sensitivity of the issue.

"It's not a done deal yet, but the United States is definitely extending itself to try to get to Yes," the first diplomat said of possible US participation in nuclear talks.

He said Nick Burns, US undersecretary of state for political affairs, would represent the United States at such talks.

Earlier Iran insisted that the West had to accept the "irreversibility" of its nuclear programme, as foreign powers prepared for more talks to find a solution to the escalating crisis.