WASHINGTON, Jan 14, 2010 (AFP) - The United States on Thursday said it would step up airport security measures in response to the threat from Al-Qaeda.

"We must remain vigilant about the continued threat we face from Al-Qaeda," Homeland Security Secretary Janet Napolitano said in a statement. "We are taking an additional set of aviation security precautions to protect the
American people.

"Some of these measures include enhanced random screening, additional federal air marshals on certain routes and adding individuals of concern to our terrorist watch list system."

The announcement comes after President Barack Obama launched a wide-ranging investigation into the attempted bombing of a trans-Atlantic jet on Christmas day.

US lawmakers have been sharply critical of intelligence failures that allowed a young Nigerian man to board a crowded Detroit-bound airliner, allegedly looking to blow it up with a potent explosive sewn into his
underwear.

Umar Farouk Abdulmutallab, 23, has been charged with attempted murder and trying to use a weapon of mass destruction on board a Northwest plane carrying 290 people from Amsterdam to Detroit.

The attack, which is thought to have been hatched in Yemen, has also brought Al-Qaeda's operations in that Arabian Gulf state into sharp focus.

"We are facing a determined enemy" Napolitiano said, warning that travelers visiting the United States should expected longer airport delays.

"As a result of these measures and others we have put in place since Christmas, travelers should allot extra time when flying - particularly into the United States from overseas."

Obama had already unveiled measures to strengthen US watchlists as well as new steps to boost airport screening and bomb detection after the botched attack.

While laborious airport screening irks many foreign visitors to the United States, tougher measures may not be unpopular with Americans, according to a series of polls released since the incident.

On Thursday, a Quinnipiac University survey showed 63 percent of respondents said US anti-terror policies leaned too far toward protecting civil rights rather than national security.

Eighty-four percent supported greater use of airport body scanners, despite the privacy concerns voiced by some opponents.