In a phone call on August 4 among their envoys handling the nuclear talks, the six powers agreed that “in the absence of a clear, positive response from Iran, we have no choice but to pursue other measures against Iran,” acting US State Department spokesman, Gonzo Gallegos, said.

The Security Council had offered the incentives two weeks ago, and August 2 had been the deadline for Iran to respond.

Iran's top nuclear negotiator, Saeed Jalili, told European Union foreign policy chief, Javier Solana, who is also the lead European negotiator, that Iran would provide a written response to the offer on August 5.

It was not known whether a late response from Iran would head off the sanctions.

The council had offered to suspend sanctions against Tehran in exchange for a suspension of its uranium enrichment activity, which the international community fears could contribute to the production of a nuclear weapon.

US Secretary of State Condoleezza Rice last week had warned Iran not to delay its response to the incentive offer, made by Security Council permanent members the United States, Britain, France, Russia and China. Germany also joined in the offer to Iran.