Military jets were patrolling major US cities and missile batteries deployed in the capital as officials said Washington, DC, New York, Los Angles, Las Vegas and San Francisco were at the "greatest risk" of being attacked by terrorists during the holiday season.
American security officials also sent out alerts to Saudi Arabia, Bahrain, Yemen and Kenya that the Al Qaeda might be planning attacks, especially against Americans interests.
Officials said on Tuesday that Los Angles and its international airport appeared to be at the greatest risk and police patrols were intensified around the perimeter of the airport because of fears that terrorists might try to shoot down an airliners using shoulder launched missile.
American officials said their major worry was that terrorists might hijack some foreign airliner to use it as weapon.
They were worried about the effectiveness of background checks on pilots carried out by foreign airlines.
The Los Angles police arrested one person in, what was described as, ongoing effort to disrupt terrorist activities but the man was charged with fraud and not terrorism.
CNN quoted an official as saying that "massive investigative effort" had been launched in the area to "beat the ticking clock".
