U.S. officials are defending the claim by Secretary of State Colin Powell that Iran is trying to modify some missiles to accommodate nuclear warheads.

At the regular State Department briefing Friday, Deputy spokesman Adam Ereli said Mr. Powell's statements about Iran's nuclear ambitions were correct. "We believe that we are on very, very solid ground in pointing to a clandestine effort by Iran to develop weapons of mass destruction and their delivery systems," he said.

Speaking while enroute to a summit in Chile, Mr. Powell said Wednesday that Iran is working to retro-fit ballistic missiles to carry a nuclear payload. Analysts say such information, if true, is significant. But the Washington Post reported that the intelligence cited by Mr. Powell has not been verified. Doubt was also cast on other claims made by an Iranian exile opposition group about Iran's nuclear program.

Mr. Ereli was grilled by skeptical reporters, several of whom alluded to having been misled before about claims of weapons of mass destruction in Iraq. But he said Mr. Powell did not make a mistake.

"I don't think there's a question of backing up claims or not backing up claims," he said. "The secretary did not misspeak. The secretary knows exactly what he was talking about, and there is a firm basis for the secretary making the remarks he made. And there should be, I think, no question in our mind about casting doubt."

Iran dismissed Mr. Powell's claim as "baseless" and says it is not pursuing nuclear weapons, only nuclear energy. Mr. Ereli says, however, that it is not believable that a country with the world's second largest reserves of natural gas would need nuclear power.

Britain, France, and Germany recently reached agreement with Iran to curb Tehran's nuclear activities in return for trade and other concessions. The pact, which goes into effect Monday, calls for Iran to suspend uranium enrichment.

However, diplomats in Europe say Iran is working quickly to produce uranium hexafluoride gas before the Monday deadline. When converted in centrifuges, the gas can be used for peaceful nuclear energy or to produce weapons-grade uranium for nuclear warheads.

Experts note that it is relatively simple to just produce an atomic bomb, but it is far more difficult to produce nuclear warheads that can be carried to a target by missiles. Although Western intelligence estimates about the pace of Iran's nuclear program vary, most agree Iran is three to five years away from building a nuclear bomb.