"Hard working Americans will be working over the holiday season to do everything we can to protect Americans from harm," Bush said during a ceremony in which a Menorah was lighted in the White House to celebrate the Jewish Hanukkah festival.

"American citizens need to go about their lives but as they do so they need to know that governments at all levels are working as hard as we possibly can to protect the American citizen."

Earlier in the day the US president met with his top anti-terror advisors after the government on Sunday raised its nationwide security level from yellow, denoting "elevated" risk, to orange, or "high" risk, amid concerns of an impending domestic terrorist attack.

Bush called a meeting of the Homeland Security Council, including officials from the Justice Department and the Federal Bureau of Investigation (FBI), as organizations and cities nationwide stepped up their security levels.

Homeland Security Secretary Tom Ridge said that at the meeting "we reviewed the specific plans and the specific action we have taken and will continue to take."

Ridge and other top officials said the threat of a devastating year-end attack by Osama bin Laden's al-Qaeda network was the most serious since the terrorist strikes of September 11, 2001.

In television broadcasts blanketing US airwaves Monday, Ridge said terrorists might attempt to use airplanes to carry out a strike and analysis that came after reviewing recent monitored communications or "chatter" among terror groups.

Security has also been boosted at airports, border crossings, shopping centers and other sites where large crowds are likely to gather this week for holiday events.

The North American Aerospace Defense Command and US Northern Command issued a statement Monday to say "appropriate action" had been taken to ensure people's protection.

"In an increased threat environment, we may take actions such as increasing our fighter presence, both alert and airborne around the nation," said spokesman for both Norad and Northcom Mike Perini.

The spokesman said that since the September 11 attacks, Norad on more than 1,600 occasions scrambled fighters or diverted air patrols to respond to possible threats.

Ridge said terrorists hoped to "rival or exceed the scope and impact" of the September 11 attacks, which left nearly 3,000 people dead. On Monday he told CNN that authorities had received the tip-off "from many, many sources from around the world."

Meanwhile, police closed a busy downtown Washington street for several hours Monday after receiving a report of potentially hazardous materials in a dumpster but the discovery turned out to be a false alarm.

Several blocks of 15th Street were shut down to traffic after a "mysterious white powder" was found in the dumpster, a police officer on the scene said. The substance turned out to be harmless, he added.