"Iran is dangerous," Mr. Bush said after a White House meeting with Italian President Giorgio Napolitano. "We believe Iran had a secret military weapons program, and Iran must explain to the world why they had such a program."
President Bush's comments came after Iranian President Mahmoud Ahmadinejad said that it was "a step forward" that U.S. intelligence agencies had concluded that Tehran stopped developing its nuclear weapons program four years ago.
Ahmadinejad told reporters that an "entirely different" situation between the United States and Iran could be created if more steps like the intelligence report followed.
"We consider this measure by the U.S. government a positive step. It is a step forward," Ahmadinejad said.
"If one or two other steps are taken, the issues we have in front of us will be entirely different and will lose their complexity, and the way will be open for the resolution of basic issues in the region and in dealings between the two sides," he said.
Iran has said its nuclear program is peaceful, but until last week, the United States and Western allies had countered that Iran was hiding plans for a bomb.
"Iran has an obligation to explain to the IAEA why they hid this program from them," President Bush said, referring to the nuclear watchdog International Atomic Energy Agency.
"Iran is dangerous, and they'll be even more dangerous if they learn how to enrich uranium," Mr. Bush said. "So I look forward to working with the (Italian) president to explain our strategy and to figure out ways we can work together to prevent this from happening for the sake of world peace."
The president's comments amounted to a renewed effort to keep pressure on Iran after the release of last week's National Intelligence Estimate. That report found that Iran had halted its nuclear weapons program four years ago, and administration officials worry it could weaken their ability to build global pressure on Tehran to stop its uranium enrichment program.
Israel warned Tuesday that Iran's nuclear program is dangerous and called for international action to prevent Iran from acquiring nuclear weapons.
In his first comment about Iran since last week's release of the U.S. intelligence report, Prime Minister Ehud Olmert said "Iran was and remains dangerous, and we must continue international pressure with full force to dissuade Iran from nuclear tendencies."
He said Iran has no need for electricity produced by nuclear energy, "doesn't have the infrastructure to create energy for civilian purposes, does not need to act with frenzied haste to create enriched uranium unless it wants to develop nuclear weapons."
Meanwhile, an exiled Iranian opposition group on Tuesday also contested the U.S. intelligence report, insisting the bomb-making program resumed the following year.
"We announce vehemently that the clerical regime is currently continuing its drive to obtain nuclear weapons," said Mohammad Mohaddessin, a spokesman for the Paris-based National Council of Resistance of Iran, or NCRI.
Mohaddessin told a news conference that Iran appeared to have duped U.S. intelligence into that conclusion.
"The clerical regime leaks false information and intelligence to Western intelligence services, through double agents," he said.
Mohaddessin said Iran did shut down a Tehran weapons program center known as Lavizan-Shian in 2003 under international pressure and demolished the site. However, Mohaddessin claimed the Iranian authorities shifted their weapons program to other sites, which resumed the work in 2004.
The NCRI is the political wing of the People's Mujahedeen of Iran, an opposition group that advocates the overthrow of government in Tehran. The Mujahedeen have been designated a terrorist group by Iran and by both the United States and the European Union.
It was not possible to independently verify the NCRI claims, which Mohaddessin said came from sources within Iran, including some among staff at covert nuclear plants.
Four years ago, the group disclosed information about two hidden nuclear sites that helped uncover nearly two decades of covert Iranian atomic activity. But much of the information it has presented since then to back up claims that Iran has a secret weapons program has not been publicly verified.