Passengers aboard international flights traveling to or through the United States will face even tighter security following an al Qaeda threat from Yemen.

A U.S. counterterrorism official said American intelligence agencies are intensely examining all information about the threats, including potential plots and specific individuals

The U.S. has increased the number of air marshals on international flights and added hundreds of names to the terrorist watch list since the attempted Christmas Day bombing on the Northwest Airliner in Detroit, Mich.

Homeland Security Secretary Janet Napolitano has urged international passengers to arrive at their respective airports even earlier for their flights.

"We are facing a determined enemy and we appreciate the patience of all Americans and visitors to our country, and the co-operation of our international partners as well as a committed airline industry," she said.

President Barack Obama was briefed Thursday by his national security team on progress against al Qaeda and its affiliates.