FBI director Louis Freeh said: "The FBI must respond to changing threats and needs that involve foreign counter-intelligence, espionage, terrorism, domestic preparedness and infrastructure protection."

As part of the reorganisation, reported to be one of the biggest in the past decade, the FBI has set up a new department which would work specifically to prevent terrorist attacks instead of merely responding to them.

Louis Freeh: Protecting the US from terrorism is one of FBI's highest priorities

Spokesman Jim Davis said the anti-terrorist unit "should allow us to increase our efficiency to deal with these threats".

Another new office would coordinate the FBI's international activities, gathering its own intelligence and working with the counter-espionage agencies of other countries.

FBI agents have increasingly been called upon to investigate cases outside the US, such as the bombing of its embassies in Nairobi and Dar-es-Salaam.

Mr Freeh said both new departments were "essential" in terms of preventing or minimising "potential harm from terrorism and foreign intelligence activities".

He said: "Protecting America from terrorism and ensuring our national security are the FBI's highest priorities."

'Sea change'

FBI deputy director Robert Bryant described the reorganisation as a "sea change" in Thursday's Washington Post.

As well as helping to prevent terrorist attacks, the reorganisation will also help agents in different divisions share information more easily and co-ordinate their efforts, he said.

"We collect a lot of information, but ... our predictive intelligence is poor. The bottom line is 'we don't know what we know'," Mr Bryant told the paper.

The FBI said its counter-terrorism budget had tripled over the past three years, while its overall budget has only doubled from $1.5bn to $3bn since 1993.