"Iran with a nuclear weapon would be a very destabilizing force in the world," he said during a Wednesday photo opportunity at the White House with Polish President Aleksander Kwasniewski.
"The Iranians just need to know that the free world is working together to send a very clear message: Don't develop a nuclear weapon," the president added.
Iran has admitted concealing some nuclear development programs and facilities, but insists they were for peaceful energy purposes and not enriching weapons-grade materials.
During recent talks with Britain, France and Germany, Iran agreed to temporarily halt its nuclear programs while negotiating with the international community on trade and other financial incentives for permanently stopping projects that could be used in weapons development.
Iranian President Mohammad Khatami, however, now says Iran would never completely forswear nuclear material enrichment capabilities.
The United States does not have diplomatic relations with Iran and has approved European allies acting for it in the Iran matter.
The United States threatens to take Iran to the U.N. Security Council over its earlier disregard of international mandates on nuclear development, which could result in worldwide economic sanctions.