"One of the reasons that we raised it is that because we believe the threat has substantially increased in the last couple of weeks," Ridge said on the CBS News Early Show.
It's "the most significant set of warnings since prior to Sept. 11," he said. "The threat is real. The threat is real."
The administration last Friday, citing intelligence that it said suggested a growing threat from Osama bin Laden's al Qaeda terrorist network, increased the level of alert from yellow to orange.
On the five-step alert scale, red is the highest, but no such terrorist warning level has yet been issued.
Police stepped up security at airports, subways and hotels in the New York metropolitan area this past weekend after the nation was put on heightened terrorist alert and law enforcement officials indicated the city was a possible target.
Stepped-up protections stretched across the country, from the San Francisco-Oakland Bay Bridge to the NBA All-Star Game in Atlanta on Sunday. Nuclear facilities in California and Washington state and some utilities put security plans into action.
Asked Monday what U.S. citizens are expected to do in response to such warnings, Ridge said, "When we raise the level of alert, when we raise the national consciousness about the level of attack, that in itself, is a deterrence. ... Just being more ready, being more prepared, is a deterrent in and of itself."
Asked about critics' accusations that the alert might have been tied to President Bush's warning to Saddam Hussein that time is running out on Baghdad avoiding war, Ridge said, "Well, I regret that interpretation."
Appearing on another news program Monday, Ridge said the warning was based on "the accumulation of credible corroborated sources, none of which are connected to the possibility of military involvement with Iraq."
Ridge, however, said it was not possible to be more specific about possible targets.
"We get general information and specific information, but none of the specific information talks about time, place or methods or means ... We don't get the specificity that we would all like to have in order to prepare," he said.