LONDON, Nov. 5 (Xinhua) -- The new head of British security service MI5 Jonathan Evans said on Monday that there are at least 2,000 people in Britain who pose a threat to national security because of their support for terrorism.

    In his first public speech, made in Manchester, since taking the job in April, Evans said there had been a rise of 400 since November 2006 and some are young as 15.

    Calling Islamic extremism the "most immediate and acute peacetime threat" in the 98-year history of MI5, he said "The more that this ideology spreads in our communities, the harder it will be to maintain the kind of society that the vast majority of us wish to live in."

    "As I speak terrorists are targeting young people and children in this country," he said, "They are radicalizing, indoctrinating and grooming young, vulnerable people to carry out acts of terrorism."

    "This year, we have seen individuals as young as 15 and 16 implicated in terrorist-related activity," he said.

    "Al-Qaida has a clear determination to mount terrorist attacks against the United Kingdom," he said. "This remains the case today, and there is no sign of it reducing."

    In Iraq, Algeria and parts of East Africa, especially Somalia, he said, the "al-Qaida brand" had expanded and now posed a threat to Britain.

    Evans also warned about the number of Russian spies in Britain. "Since the end of the Cold War we have seen a no decrease in the numbers of undeclared Russian intelligence officers in UK -- at the Russian Embassy and associated organizations -- conducting covert activity in this country," he added.