Interrogating detained Al Qaeda terrorists at the U.S. military base in Kandahar has prevented new attacks against U.S. targets worldwide, the FBI director said Wednesday.

Robert Mueller made an unannounced visit to the base, the largest concentration of U.S. forces in Afghanistan. As many as 400 Al Qaeda and Taliban prisoners have been held at the base since the Taliban regime collapsed under attack by U.S.-led forces in November.

"Information we have picked up since the war has prevented additional attacks around the world," Mueller said. "Interrogations from Al Qaeda members detained here in Afghanistan as well as documents ... has prevented additional attacks against U.S. facilities around the world."

Mueller ate a lunch of Afghan chicken and rice with FBI agents who are here to interrogate detainees. They are the first agents deployed in a combat zone.

Mueller refused to give details on what attacks may have been prevented. But Singapore authorities last month arrested suspects they said were plotting attacks against the U.S. Embassy and other targets, based partly on intelligence picked up from Afghanistan.

The FBI director said that he was traveling to find out "what more needs to be done to assist FBI agents who have participated with Army and other forces here to learn all we could and can about terrorist activities."

Mueller said he could not say anything about the military's continuing hunt for Usama bin Laden or his chief Afghan ally, deposed Taliban leader Mullah Mohammed Omar.

U.S. special forces and Afghan anti-Taliban fighters conducted house-to-house searches in four villages in southern Helmand province seeking Omar, the one-eyed cleric whose extreme Islamic regime allowed bin Laden to use the country as a base, Afghan sources said.

Neither Omar nor his aides were discovered, the sources said on condition of anonymity. U.S. officials refuse to confirm special forces operations.

In Germany, where three suicide hijackers lived undetected before staging the Sept. 11 attacks in the United States, an intelligence official said that Al Qaeda had suffered a major blow from the military campaign and may not be able to stage major new operations.

Al Qaeda may still be able to carry out small attacks or activate long-established plans, said Dieter Kaundinya, head of the anti-terrorism unit at Germany's foreign intelligence service. But its resources are too stretched to support big new actions, he told the Mitteldeutsche Zeitung.

The Associated Press contributed to this report.