Speaking to US Merchant Marines in New York, the US president said Iran should abandon any attempt to develop nuclear weapons and comply with international regulations.
"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 on Monday.
Bush's remarks came a day before he leaves for a US-European Union summit in Vienna where he will meet EU officials involved in negotiations with Iran.
The package, agreed upon by the five permanent members of the UN Security Council and Germany, offers Iran incentives in return for the suspension of uranium enrichment.
Tehran has so far refused to halt its enrichment programme, saying it will only use the technology to generate electricity, although it has welcomed the incentives package.
Bush 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.
"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," he added
The six countries have given Iran until mid-July to respond to their offer.