The State Department and White House reiterated that Iran must suspend all uranium enrichment on its soil as a condition for Washington's participation in negotiations with the Islamic republic.

"The precondition of suspending uranium enrichment-related and reprocessing activities -- that is still an absolute condition," said President George W. Bush's spokesman, Tony Snow.

"That condition would have to hold throughout any negotiating term," said State Department spokesman Sean McCormack.

"Beyond that, I am not going to speculate. Beyond that, we are truly into the realm of the hypothetical and theoretical," he said.

According to diplomatic sources in Vienna and Tehran, the powers' offer to Iran would eventually allow uranium enrichment on its territory, but only after the approval of the international community.

Iran on Wednesday was weighing an international incentive package -- which offers trade, diplomatic and technology incentives in return for a freeze of uranium enrichment -- drawn up by Britain, France and Germany and backed by the United States, Russia and China.

Iran insists its nuclear program is designed purely to generate electricity but the United States and others are concerned Tehran is secretly seeking to build nuclear weapons.