Chertoff: Terrorism Prevention Efforts Successful

Chertoff also highlighted what he said was the department's effectiveness in keeping would-be attackers at bay.

"It's not impossible, but for terrorists who typically operate in being very careful because they don't want their plots disrupted, we have made it harder for them to come in," he said.

Chertoff likened the reaction to a car thief who passes over a locked vehicle set with an alarm and anti-theft devices and decides to steal the one next to it.

"One of the reasons we're seeing more attacks in Europe is because they think it's easier," he said, citing almost annual attacks since 2004 in Madrid, London and Glasgow and disrupted plots in Denmark, Germany, Italy, France and Portugal.

However, the intention of al-Qaeda and affiliated groups to strike at U.S. targets has not diminished, Chertoff warned, repeating U.S. intelligence assessments last year. For example, a disrupted Britain-based plot to smuggle liquid explosives onto transatlantic airliners in 2006 would have caused deaths on the scale of the Sept. 11 attacks, he said.

While al-Qaeda's capability is "uneven" and less than what it was before 2001, it is rebounding somewhat in the frontier areas of Pakistan, he said.

Chertoff's remarks come as Washington is pushing 27 of its European and other allies to accept tighter security requirements on travelers who can now visit the United States as tourists without visas. The restrictions include electronic check-in with authorities days before travel.

He also warned against complacency, saying al Qaeda inspired extremists are "continuing to refine themselves and improve themselves. If we don't do more than we're doing, if we stop, eventually that risk is going to start to increase."