Experiments with high explosives, possibly linked to future nuclear weapons tests, were carried out as recently as 2003 in Iran, sources tell CBS News.
International Atomic Energy Agency analysts said they suspect the experiments took place at a huge military complex south of Tehran. Inspectors were permitted only one visit, and saw only part of the site, reports CBS News correspondent Sheila MacVicar.
Despite the lack of access, Sean McCormack, a U.S. State Department spokesman, said, "we are seeing more and more indications" that Iran's enrichment activities have the intended purpose of building a nuclear weapon.
Meanwhile, Iran ratcheted up its confrontation with the West on Wednesday, with its president lashing out at the United States and Europe as "bully countries" a day before a key meeting that could put Iran before the U.N. Security Council.
Tehran's top nuclear negotiator said Iran would resume large-scale uranium enrichment in response, warning that its main enrichment plant at Natanz was ready for full operation.
Iran provoked an outcry on Jan. 10 when it broke U.N. seals at the facility to begin research-level enrichment, a process that can produce material for nuclear reactors or, if sufficiently processed, atomic weapons.
"Natanz is ready for work. We only need to notify the IAEA that we are resuming (large-scale) enrichment. When we do that is our call. If they (report Iran to the Security Council), we will do it quickly," negotiator Ali Larijani said.
Earlier Wednesday, Iranian President Mahmoud Ahmadinejad derided the United States as a "hollow superpower" and vowed to pursue the Iran's nuclear program no matter what.
"Nuclear energy is our right, and we will resist until this right is fully realized," Ahmadinejad told a crowd of thousands in the southern Iran city of Bushehr, where Russia is finishing the construction of Iran's first nuclear power plant.
"Our nation can't give in to the coercion of some bully countries who imagine they are the whole world," he added.
The crowd responded with chants of "Nuclear energy is our right," CBS radio correspondent Angus McDowell reports.
In an interview with the AP on Wednesday, Mr. Bush repeated his opposition to an Iranian nuclear capability.
"We cannot afford to have Iran with a nuclear weapon," the president said. "We want them to have nuclear power but under the conditions that we describe."
The IAEA, the U.N.'s nuclear watchdog group, said it has found "administrative interconnections" between uranium enrichment, the high explosives tests and the design of a missile warhead, all of which could have a "military-nuclear dimension," MacVicar reports.
Mr. Bush held out little hope of avoiding a showdown with Tehran. "It looks like to me the process is headed toward the (U.N.) Security Council, and that if the Iranians would like to avoid that, they ought to work in good faith to get rid of their nuclear weapon ambitions," the president said.
Mr. Bush's comments were echoed throughout Washington Wednesday.
"A nuclear armed Iran committed to the extinction of Israel … is a serious threat to global security," Sen. John McCain, R-Ariz., said on CBS News' The Early Show. "It's probably the greatest single challenge outside of the war on terror that we faced since the end of the cold war."
"What we're doing now is bringing the entire pressure of the entire world on Iran," Sen. Joseph Biden, D.-Del., told the The Early Show. "And if it fails, then the whole world is in on the deal."
President Bush discussed Iran in a telephone call Wednesday with Russian President Vladimir Putin and thanked him for Russia's offer to divert enrichment activities to Russia in an effort to keep an eye on the process and make sure they are designed for civilian use.
"They both agreed that it was important to stay in close contact as we move forward to address this issue," White House spokesman Scott McClellan said. "I think both leaders have a shared concern about Iran developing a nuclear weapon under the guise of a civilian program."
The administration wants Russia to take a tough line on Iran at the United Nations, and officials have suggested Russia, for geographic reasons, has at least as much cause for concern as the United States.
But Russia is conflicted, wanting also to preserve its commercial and military ties to Iran.
The IAEA's 35-nation board of governors is to meet in Vienna, Austria, Thursday, and is expected to report Iran's nuclear program to the U.N. Security Council.
The five permanent members of the Security Council agreed Tuesday that Iran should be put before the world body.
The Security Council has the power to impose sanctions, but such a move is not likely soon. Under the deal the United States, Britain and France made with Moscow and Beijing — who tend to support Iran — the council will likely await a new IAEA report on Iran in March before deciding on substantive action.
Larijani said Iran was "committed to the Nuclear Nonproliferation Treaty" but would end IAEA snap inspections of its facilities if the Thursday vote went against it.
"The result would be Iran's cooperating with the IAEA at a low level, which is against our wishes. All our suspensions on nuclear activities would be lifted," he said.
"The law obliges the government" to end voluntary suspension of nuclear activities ... "naturally, uranium enrichment at industrial scale" would resume, he said.
Last year, Iran's parliament passed a law obliging the government to resume full-scale nuclear activities if the country was taken to the Security Council.
Iran insists its nuclear program is designed for electricity generation. But on Tuesday, the IAEA said that Iran had turned over to it documents and drawings it obtained on the black market that could serve no other purpose than production of an atomic warhead.