The United States warned Thursday that Iran's continued uranium enrichment meant that any new offer by world powers on its nuclear program would be more burdensome than one it had already rejected.

White House spokesman Robert Gibbs said increased expectations required for any deal reflected the fact that Iran's enriched uranium stocks were now larger than they were when previous talks broke down last year.

"Based on the unilateral actions that they took, they have increased their enrichment," Gibbs said.

"In order to live up to the responsibilities that they have made and to lift any sanctions, they would have great responsibilities," Gibbs said.

"The responsibilities get greater each and every day even as the sanctions impact their economy more and more."

Gibbs spoke after the New York Times reported that the Obama administration and its European allies were preparing a new, more onerous offer for Iran than the one rejected by supreme leader Ayatollah Ali Khamenei last year.

The offer would require Iran to send more than 4,400 pounds of (1,995 kilograms) of low-enriched uranium out of the country, an increase of more than two-thirds from the amount required under a deal struck in Vienna.

"This will be a first sounding about whether the Iranians still think they can tough it out or are ready to negotiate," an unnamed senior American official told the newspaper.

"We have to convince them that life will get worse, not better, if they don't begin to move."

Another senior US official said the United States and its European partners were "very close to having an agreement" to present to Iran.

But the Islamic republic has yet to respond to a request by EU foreign policy chief Catherine Ashton, who represents world powers in the nuclear dialogue with Iran, to meet in Vienna in mid-November.

The Times said many US officials suspect the new initiative under development is likely to fail, but would fulfill US President Barack Obama's promise to keep negotiating even while the pressure of sanctions increases.

Iran has signaled it is ready to discuss a possible exchange of atomic fuel at the upcoming talks for a Tehran-based research reactor after consultations broke down last year between the Islamic republic and the Vienna group comprising France, Russia, the United States and the UN atomic watchdog.

Under an initial proposal brokered by the International Atomic Energy Agency, Iran would send more than 2,600 pounds (1,200 kilograms) of its low-enriched uranium to Russia and France for conversion into the fuel rods required for the Tehran reactor.

In May, Iran responded by its own counter-proposal brokered by Turkey and Brazil, which was cold-shouldered by the West before the United Nations Security Council slapped a fresh round of sanctions on Tehran less than a month later. Several countries imposed further unilateral sanctions.

earlier related report
Iran accepts nuclear talks 'after' November 10: EU's Ashton
Brussels (AFP) Oct 29, 2010 - EU chief diplomat Catherine Ashton said on Friday that Iran had accepted the principle of re-starting talks on its disputed nuclear drive "after" November 10.

Iran's negotiator with the West, Saeed Jalili, "is willing to agree a date and a time after the 10th of November to begin the dialogue," Ashton said on her arrival at a European Union summit in Brussels.

Jalili "wants to agree a time and place," she said, noting that she had received a letter from Jalili on Friday morning.

"I think it's a significant move and we're now in touch with Iran to see if we can agree the time and the place which is possible."

The United States warned Thursday that Iran's continued uranium enrichment meant that any new offer by world powers on its nuclear programme would be more burdensome than one it had already rejected.

Ashton, who represents world powers in the nuclear dialogue with Iran, has proposed a meeting in Vienna in mid-November.