The United States went on high alert for a possible terror attack as Homeland Security Secretary Tom Ridge (search) said that threat indicators are "perhaps greater now than at any point" since Sept. 11, 2001.

"Extensive and considerable protections have been or soon will be in place all across the country," Ridge said on Sunday in Washington as he announced that the threat level would rise from yellow, or elevated, to orange, the second-highest level.

"Your government will stand at the ready 24 hours a day, seven days a week, to stop terrorism during the holiday season and beyond."

Americans were told to stick to their travel plans despite intelligence indicating the Al Qaeda (search) terrorist network is seeking again to use planes as weapons and exploit suspected weakness in U.S. aviation security.

Some of the intelligence information gathered in the past few days suggests that "extremists abroad" are anticipating attacks that will rival or exceed the scope of those of Sept. 11, Ridge said.

He also said officials did not see a connection between last weekend's capture of ousted Iraqi leader Saddam Hussein (search) and the heightened security alert.

The threat information comes from multiple, credible sources but officials are unaware of a specific target or means of attack, added a senior law enforcement official.

Some of the intercepted communications and other intelligence mentions New York, Washington and unspecified cities on the West Coast, said the official, who spoke on condition of anonymity. Authorities also are concerned about dams, bridges, nuclear plants, chemical facilities and other public works.

Thousands of state and local law enforcement agencies have received an FBI advisory urging special notice of sites that could be a conceivable target and potential security upgrades, the official said.

In addition, Ridge has contacted his counterparts in Canada and Mexico about increasing border security.

The State Department issued a worldwide caution to U.S. citizens overseas. "Al Qaeda and its associated organizations have struck in the Middle East in Riyadh, Saudi Arabia, and in Europe in Istanbul, Turkey," the department's Web site said. "We therefore assess that other geographic locations could be venues for the next round of attacks.

"We expect Al Qaeda will strive for new attacks designed to be more devastating than the Sept. 11 attack, possibly involving nonconventional weapons such as chemical or biological agents," the State Department advised.

At the hastily arranged news conference at Homeland Security headquarters, Ridge said credible intelligence sources "suggest the possibility of attacks against the homeland around the holiday season and beyond."

"The strategic indicators, including Al Qaeda's continued desire to carry out attacks against our homeland, are perhaps greater now than at any point since Sept. 11," he said.

The alert level had stood at yellow, an elevated risk and in the middle of the five-color scale, since May.

The White House declined comment, referring all questions to Ridge's department.

Ridge said the government acted after U.S. intelligence agencies "received a substantial increase in the volume of threat-related intelligence reports."

"Recent reporting reiterates that Al Qaeda continues to consider using aircraft as a weapon. They are evaluating procedures both here and abroad to find gaps in our security posture that can be exploited," Ridge said.

But he added that U.S. aviation "is far more secure" than ever.

As a result of the change in threat level, all federal departments and agencies were putting action plans in place and stepping up security at airports, border crossings and ports, Ridge said.

The secretary sought to reassure Americans about the warning, urging them not to disrupt holiday plans and to use common sense and report suspicious items and to prepare or review personal emergency plans.

"We have not raised the threat level in this country for six months, but we have raised it before. And as before, Americans can be assured that we know what we must do and we are doing it," Ridge said.

Federal aviation security officials were in contact Sunday with general aviation officials, including the Aircraft Owners and Pilots Association, which represents 400,000, or two-thirds of all pilots in the United States.

U.S. officials by the end of last week were telling holiday travelers to be vigilant about the threat of attacks. The warning was prompted in part by a raised level of ominous intercepted communications that has not quieted for months.

On Friday, the Arabic television network Al-Jazeera aired a new statement from Ayman al-Zawahri, bin Laden's chief deputy. The CIA said Saturday it believes the tape is authentic.

"We are still chasing the Americans and their allies everywhere, even in their homeland," according to the voice on the tape.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said earlier Sunday that officials were trying to determine whether the increased material detected was an aberration or something more serious.

"There is no doubt, from all the intelligence we pick up from Al Qaeda, that they want to do away with our way of life," he told "Fox News Sunday" after his return from a trip to Iraq and Afghanistan.

Four times before had the threat level risen to orange. Each change sets off a flurry of increased security measures by cities, states and businesses. The lowest two levels, green and blue, and the highest, red, have not been used since the system was put in place in early 2002.

Fox News' Anna Stolley and The Associated Press contributed to this report.