"And there's still substantial threat out of Yemen," he said. "And now you have the countries in the Arab Spring: Tunisia, Libya, Syria, Mali; Egypt most recently, where they're breeding grounds for radical extremists who may not stay there, but may present an attack."

Mueller, who was charged with reshaping the FBI from a domestic crime-fighting force into a counterintelligence service with a key role in combating terrorism, also spoke of the homegrown extremists with in the US.

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"And, finally, you have, within the United States, the growth of homegrown, radicalised extremists who are radicalised on the Internet and then get their instructions for developing explosives on the Internet, as well," he said.

Mueller, who was appointed director by Republican President George W Bush, had his term extended by Democratic President Barack Obama with congressional approval beyond the normal 10-year maximum.

Mueller also expressed the view that the kind of intelligence being collected today by the National Security Agency (NSA) could have prevented at least a part of the 9/11 attack.

"I think there's a good chance we would have prevented at least a part of 9/11. In other words, there were four planes," he said.

"There were almost 20 -- 19 persons involved. I think we would have had a much better chance of identifying those individuals who were contemplating that attack," Mueller said.

Mueller also did not foresee the day when the US arms drones would be used to take out individuals who are posing threats to Americans on American soil.

"We have used it a handful of times to provide surveillance in tactical situations where we, for instance, have a hostage rescue operation undergoing and for very narrow tactical purposes in limited situations," he said.

Asked if America was in the day where Big Brother is now present in Americans' lives, Mueller said: "I wouldn't go so far as to say that at all."

The controversial data collection programmes, on the other hand, he said had made America "hugely" safer.

Asked about the Boston bombing despite such programmes, Mueller countered:

"The number of plots that we have disrupted would be in the dozens, and not just disrupted here but disrupted overseas."

"The ability now to work and exchange information with the CIA, with NSA, with our counterparts in the intelligence services and law enforcement entities overseas has grown substantially. And that makes us all safer," he said.u00a0