Washington ? Secretary of State Colin Powell shared information with reporters Wednesday about Iran’s nuclear program that was classified and based on an unvetted, single source who provided information that two U.S. officials said Thursday was highly significant if true but has not yet been verified.

Powell and other senior Cabinet members were briefed last week on the sensitive intelligence. The material was stamped “No Foreign,” meaning it was not to be shared with allies, although President Bush decided that portions could be shared last week with British Prime Minister Tony Blair, officials said.

According to one official with access to the material, a “walk-in” source approached U.S intelligence earlier this month with more than 1,000 pages purported to be Iranian drawings and technical documents, including a nuclear warhead design and modifications enabling Iranian ballistic missiles to deliver an atomic strike. The official agreed to discuss the information on the condition of anonymity and only because Powell had alluded to it publicly.

But U.S. intelligence officials have been combing the information carefully and with a wary eye, mindful of the mistakes made in trusting intelligence information alleging that Iraq possessed weapons of mass destruction. Powell, who announced earlier this week that he would not stay on for a second term, presented that intelligence in a February 2003 speech to the U.N. Security Council that was meant to convince the world that Saddam Hussein needed to be forcefully removed from power. Much of his presentation turned out to be based on information provided by unreliable sources.

The official said the CIA remained unsure about the authenticity of the Iran documents and how the informant came into their possession. A second official would say only that there are questions about the source of the information.