Facing what one official says is the most serious threat to American soil since the Cuban missile crisis, U.S. officials have banned private aircraft from the airspace around nuclear reactors and ratcheted up general security across the nation.

New details have emerged about the nature and source of the threat, which suggests that a terrorist attack is imminent "in the next week or so," according to Homeland Security Director Tom Ridge. He said the national alert was based on information from "multiple sources, and they were credible sources."

U.S. officials, speaking to Fox News on the condition of anonymity, said the threats hinted that one of Usama bin Laden's lieutenants in Afghanistan recently urged new attacks on Americans. The information also suggested that known Al Qaeda operatives in Canada, Asia and elsewhere were discussing new attacks.

Based on intelligence gathered from intercepts of phone calls by Al Qaeda members, U.S. officials said they are most concerned about Wednesday to Friday this week. They said they have no specific information about the form of any possible attack, nor can they pinpoint any specific region as a likely target.

The intercepted calls were brief and filled with what appeared to be "general conversation," the officials said. They were, however, laced with code words used repeatedly in multiple calls to different individuals referring to a "big event" to "the south" this week.

Officials also received information from individuals close to Al Qaeda, but none of it from individuals rounded up and held in U.S. custody since the Sept. 11 attacks, the officials said. Significant movement in recent days by members of Al Qaeda overseas — similar to movements noted in the days leading up to Sept. 11 — only heightened the concern, the officials said.

Orders for the attacks may have come from someone in bin Laden's inner circle, possibly a top lieutenant in Afghanistan. But the officials cautioned that since Al Qaeda members know their calls and movements are being monitored, the conversations may be a form of disinformation to throw law enforcement officials off track before a real attack.

U.S. officials long have suspected that bin Laden's top deputies, Ayman al-Zawahri and Mohammed Atef, were involved in the planning or support of the Sept. 11 suicide hijackings that killed 5,000 people in New York and Washington.

There were suspicions that terrorist cells already trained or financed by Al Qaeda might be willing to act without a central order from Afghanistan, which would make it more difficult to detect where the next attacks were coming from.

Canadian Solicitor General Lawrence MacAulay, who oversees his nation's law enforcement and intelligence, said Tuesday that information his country provided to the United States was behind the warning. Information provided to the FBI from the Canadian Security Intelligence Service "led Mr. Ashcroft to make the statement that he made yesterday," MacAulay said.

Following the alert, the Federal Aviation Administration temporarily banned private planes from flying near nuclear power plants "for reasons of national security." No planes may fly within 11 miles of 86 nuclear plants and other nuclear sites such as the Sandia National Laboratory in New Mexico. The ban will expire Nov. 7.

Officials implemented the ban even though small planes are not generally considered a threat to nuclear plants.

Commercial airplanes, which fly at higher altitudes, will not be affected. Nor will the ban apply to medical, law enforcement, rescue and firefighting operations when authorized by air-traffic controllers.

Fox News' Rita Cosby and the Associated Press contributed to this report.