Bush noted in his annual State of the Union address that intense diplomacy had convinced Libya to abandon its aspirations for weapons of mass destruction -- but that "12 years of diplomacy with Iraq did not."
"For diplomacy to be effective, words must be credible, and no one can now doubt the word of America," Bush said to cheers from a joint session of Congress.
"Different threats require different strategies. Along with nations in the region, we are insisting that North Korea eliminate its nuclear program.
"America and the international community are demanding that Iran meet its commitments and not develop nuclear weapons.
"America is committed to keeping the world's most dangerous weapons out of the hands of the world's most dangerous regimes."
The United States has been locked in a nuclear showdown with North Korea since late October 2002, when it accused the Stalinist state of embarking on a program to enrich uranium in violation of a 1994 anti-nuclear deal.
It refused to pay Pyongyang to end the program, and the Stalinist state retaliated by unfreezing a separate nuclear program mothballed under the 1994 pact.
Efforts to end the crisis through diplomatic means have moved ahead only slowly, with China struggling to get agreement to convene a new round of six nation crisis talks also involving the United States, North Korea, South Korea, Russia and Japan.
Iran has denied US charges that it is secretly trying to develop nuclear weapons.
But Tehran agreed last year to heed International Atomic Energy Agency calls to suspend all enrichment related and reprocessing activity.
Some diplomats have recently warned Iran was still acquiring material to build centrifuges as it differs with the UN nuclear watchdog over how to fulfill the pledge.
Iran has insisted it is living up to all its IAEA obligations.