US President George W. Bush told Iran on Monday that nations worldwide will not back down from their demand that Tehran suspend uranium enrichment."Iran's leaders have a clear choice. We hope they will accept our offer and voluntarily suspend these activities so we can work out an agreement that will bring Iran real benefits," Bush said a day before leaving for Vienna, Austria, where he will talk with European Union officials who are leading efforts to resolve the nuclear dispute.If Iran's leaders reject the offer, they will face action before the UN Security Council and progressively stronger political and economic sanctions, Bush said during a commencement speech at the US Merchant Marine Academy.Bush discussed Iran with Russian President Vladimir Putin for about 18 minutes on Monday after Putin placed a phone call to Bush. "The presidents agreed on the importance of remaining united in their efforts to press Iran to suspend all enrichment activities and begin negotiations on the incentives package," said Kate Starr, a spokeswoman for the National Security Council.On Sunday, Iran accused the United States of trying to sway European nations from a possible compromise. The Iranian foreign ministry said U.S. insistence that negotiations be conditioned on Tehran's suspension of uranium enrichment has narrowed the scope of possible solutions, and made it more difficult for all parties to reach an accord.Bush made it clear he would not budge. He said allowing Iran to enrich uranium, a process that can make nuclear fuel for a power plant or fissile material for an atomic bomb, would present a grave threat to the world."The United States has offered to come to the table with our partners and meet with Iran's representatives as soon as the Iranian regime fully and verifiably suspends its uranium enrichment and reprocessing activities," Bush said. "I have a message for the Iranian regime: America and our partners are united. We have presented a reasonable offer. Iran's leaders should see our proposal for what it is - a historic opportunity to set their country on a better course."On June 6, European Union foreign policy chief Javier Solana presented a package of rewards and possible penalties to Iran. The package was drawn up by the five permanent members of the U.N. Security Council - the United States, Britain, China, France and Russia - and Germany.The package calls on Iran to suspend, not permanently halt, uranium enrichment as a condition for the start of talks, although the negotiations are aimed at getting Iran to agree to a long-term moratorium on such activity.Iran says enriching uranium is its country's right. Iranian officials say they are reviewing the package and will propose amendments.Bush is the first American president to address a graduating class at the academy. He spoke there at the request of former White House chief of staff, Andrew Card, who briefly studied there in the 1960s and hitched a ride on Air Force One to share the stage with the president.