British Prime Minister Tony Blair said he agreed Friday with his spy chief's assessment that the threat from terrorism is growing.

Terrorism "is serious, is growing, and will, I believe, be with us for a generation," Blair said after the head of Britain's MI5 spy agency said authorities were actively tracking 30 terrorist plots involving 1,600 individuals.

"It's a very long and deep struggle but we have to stand up and be counted for what we believe in and take the fight to those people who want to entice young people into something wicked and violent but utterly futile," Blair told journalists at his 10 Downing St. office.

The head of MI5, Dame Eliza Manningham-Buller, said authorities had foiled five major plots since the July 2005 transit bomb attacks in London, according to a transcript released by the spy agency of a speech made to a small audience of academics in London on Thursday.

Manningham-Buller said officials were "aware of numerous plots to kill people and to damage our economy," adding that many of the suspects are homegrown British terrorists plotting homicide attacks.

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"What do I mean by numerous? Five? Ten?" she said. "No, nearer 30 that we currently know of."

She said MI5 and the police were tackling 200 cells involving more than 1,600 individuals who were "actively engaged in plotting or facilitating terrorist acts here and overseas,"

Senior anti-terrorist officials have said before that they have foiled several plots since the July 2005 attacks, but Thursday's talk contained the first public estimate of the threat by the head of Britain's domestic spy agency.

Manningham-Buller, who has headed MI5 since 2002, said the plots "often have linked back to Al Qaeda in Pakistan, and through those links Al Qaeda gives guidance and training to its largely British foot soldiers here on an extensive and growing scale."

She warned that radicalization, especially of young people, was one of the biggest problems facing anti-terror investigators.

On July 7, 2005, four suicide bombers killed 52 people on three subway trains and a bus in London. Three of the four bombers were British-born.

This August, police said they had foiled a plot by a British terrorist cell to blow up trans-Atlantic airliners in midair. More than a dozen people, all British, are awaiting trial in the case.

On Tuesday, a British Muslim convert, Dhiren Barot, was sentenced to life in prison for plotting to attack U.S. financial landmarks and blow up London targets with limousines packed with gas tanks, napalm and nails.

Manningham-Buller said some of the plots MI5 was tracking could be less threatening than the deadly 2005 ones, but that they still must be investigated.

She said the threat from international terrorism "is serious, is growing, and will, I believe, be with us for a generation."

"It is the youth who are being actively targeted, groomed, radicalized and set on a path that frighteningly quickly could end in their involvement in mass murder of their fellow U.K. citizens," Manningham-Buller said. "Young teenagers are being groomed to be suicide bombers.

"Today we see the use of homemade improvised explosive devices, but I suggest tomorrow's threat will include the use of chemicals, bacteriological agents, radioactive materials and even nuclear technology," she added.

The Associated Press contributed to this report.