KRANJ, Slovenia — President Bush said today the United States and Europe must rally to keep Iran from developing a nuclear weapon, calling the threat an incredible danger to world peace.

Speaking in Slovenia at his final European Union-U.S. summit, Bush said of Iranian leaders, "They can either face isolation, or they can have better relations with all of us. ... Now's the time for all of us to work together to stop them."

Bush said the U.S. and its European allies should join forces to make that abundantly choice clear to Iran.

The president spoke as he and EU leaders were poised to threaten Iran with further financial sanctions unless it verifiably suspends its nuclear enrichment. They said Iran must fully disclose any nuclear weapons work and allow the International Atomic Energy Agency to verify that work.

In a wide-ranging news conference, Bush was asked about possible government intervention to prop up the value of the U.S. dollar. Bush essentially rejected that idea. Bush said he believed in a strong-dollar policy, but that world economies will end up setting the value of the dollar.

The summit, consisting of about three hours of meetings and a working lunch, took place in a modern glass building on the vast Brdo grounds in the shadow of Slovenia's jagged mountain peaks. The leaders addressed reporters from a lush, sun-splashed lawn next to the complex's Brdo Castle.

Bush warned that if Iran ends up with a nuclear weapon, "the free world is going to say why didn't we do something about it at the time? ... Now's the time for there to be strong diplomacy."

He said Iran "can't be trusted with enrichment."

Iran is also under fire for defying three sets of U.N. Security Council sanctions and continuing to enrich uranium — which can generate both nuclear fuel and the fissile material for the core of nuclear warheads. Iran insists that it has only civilian uses in mind for its nuclear program.

On global warming, Bush declared, "I think we can actually get an agreement on global climate change during my presidency," which ends on Jan. 20, 2009.

He said no global warming agreement can be effective without China and India.

The United States has been at odds with allies about whether any climate strategy should include mandatory emission reductions, among other sticking points. Slovenian Prime Minister Janez Jansa, the president of the European Council, said European members and the United States might have different approaches to some of its common challenges.