Washington ? The government on Sunday raised the national threat level to orange, indicating a high risk of terrorist attack, and said threat indicators were “perhaps greater now than at any point” since Sept. 11, 2001, with strikes possible during the holidays.

Americans were promised “extensive and considerable protections” throughout the country and told to stick to their travel plans despite intelligence indicating the al-Qaida 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, Homeland Security chief Tom Ridge said.

He also said officials did not see a connection between last weekend’s capture of ousted Iraqi leader Saddam Hussein 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 mention 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-Qaida 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-Qaida 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 a 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-Qaida’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-Qaida 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 are putting action plans in place and stepping up security at airports, border crossing and ports, Ridge said.

“Extensive and considerable protections have been or soon will be in place all across the country,” Ridge said.