Iran must "earn the trust of those of us who are worried about them developing a nuclear weapon," Mr. Bush said at a seaside news conference during a four-hour visit with Colombian President Alvaro Uribe.
Earlier Monday,. The head of the U.N. nuclear watchdog agency said he believed Iran had stopped its enrichment activities — the central part of an agreement with Europe designed to head off possible U.N. sanctions.
"Let's say, I hope it's true," Mr. Bush said.
But, he added, "I think the definition of truth is the willingness of the Iranian regime to allow for verification."
Iranian leaders "have said some things in the past, and it's very important for them to verify" that Tehran is not attempting to acquire nuclear weapons, Mr. Bush said.
He said that it wasn't just the United States that was concerned. "France, Great Britain and Germany and other nations around the world understand the dangers of the Iranian government having a nuclear weapon."
The United States had sought to refer Iran to the U.N. Security Council, a step that could lead to sanctions. However, if the International Atomic Energy Agency rules that Iran is honoring its commitment to suspend enrichment, it would blunt any call for sanctions.
Mr. Bush's comments on verification echoed the late President Reagan's famous "trust but verify" insistence to Soviet leaders on the subject of enforcing nuclear-arms reduction agreements.
In Vienna, the head of the U.N. nuclear watchdog agency said the suspension appeared confirmed.
"I think pretty much everything has come to a halt," Mohamed ElBaradei, the head of the IEAE, told reporters.
ElBaradei said he expected to have a definitive ruling by Thursday on whether Iran has honored its pledge made earlier this month and frozen activities that can be used in energy programs, but also to make nuclear weapons.
The United States has accused Iran of secretly pursuing nuclear weapons and has pushed the international community to take a hard line. U.S. Secretary of State Colin Powell said last week that Washington has intelligence indicating Iran is trying to fit missiles to carry nuclear weapons.
Iran, which insists its program is peaceful, has said the suspension will be brief, voluntary, and contingent on what Europe does next. Iranian hard-liners have accused the government of sacrificing Iran's rights by agreeing to suspend enrichment.
Iran radio said Monday the suspension included reprocessing uranium and building centrifuges used to enrich uranium.
In Belgium Monday, EU foreign ministers urged Iran to confirm its suspension at an IAEA board meeting in Vienna on Thursday. The board was to focus on closing an examination of nearly two decades of covert nuclear activities in Iran.
"We reached an agreement two and a half weeks ago," British Foreign Secretary Jack Straw said. "What we are looking forward to is a translation of that text into an agreement. I hope very much Iran to come fully into compliance."
Straw said that if Iran reneges on the deal the EU "reserves the right" to take Iran to the United Nations.
Only after Thursday's IAEA meeting is the EU expected to move to resume negotiations for a trade and political cooperation agreement, officials said. Such a deal would likely include help to develop a peaceful nuclear energy program.