Western powers today gave Iran just weeks to respond to a package of incentives to suspend its nuclear enrichment programme.
The offer, which EU foreign policy chief Javier Solana personally delivered to Tehran earlier this week, was prepared by Germany, France and Britain and is backed by the EU, United States, Russia and China.
US President George W. Bush said that if Iran did not stop enrichment, "there must be a consequence".
"We've given the Iranians a limited period of time - you know, weeks not months - to digest a proposal to move forward. And if they choose not to verifiably suspend their programme, then there will be action taken in the UN Security Council," Mr Bush told a news conference.
European Union president Austria earlier said Iran had until next month's Group of Eight (G8) summit to consider the offer.
Asked what would happen if Iran did not accept, Austrian Chancellor Wolfgang Schuessel told the Frankfurter Allgemeine Zeitung daily: "This will be discussed within the framework of the G8.
Iran has until the world economic summit in July to think it over." The G8 summit will be held in St. Petersburg, Russia, on July 15-17. Iran's nuclear ambitions are expected to be one of the main topics of discussion.
The comments represent the first explicit deadline for Iran to respond to the offer.