Chicago said it had asked federal authorities to change flight paths over downtown Chicago for the duration of the alert.
Experts have said the top alert -- red -- on the five-level scale would be declared only if an attack on US soil were imminent or underway.
Ridge cited a "substantial increase in the volume of threat related intelligence reports" suggesting the possibility of attacks within the US around the "holiday season and beyond" -- referring to Christmas, the Jewish celebration of Hanukkah and New Year's Day.
Due to the raised threat level, the State Department repeated a worldwide caution warning of a potential threat to Americans overseas.
On Friday, al-Jazeera television broadcast a purported audio tape by bin Laden's deputy Ayman al-Zawahri, saying al-Qaeda would target Americans everywhere, including the US.
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