The head of Germany's domestic intelligence agency says the number of Islamic extremists in the country is growing rapidly.

Hans-Georg Maassen says his agency estimates that some 6,300 people in Germany are adherents of a fundamentalist strain of Islam known as Salafism.

Maassen told rbb-Inforadio in an interview broadcast Saturday that the number of Salafis could rise to 7,000 by the end of the year, compared to about 3,800 three years ago.

He says extremist strands of Islam provide disaffected young people with a sense of belonging and purpose that allows them to hope they'll go "from being underdogs to top dogs."

Authorities estimate that some 450 Salafis have traveled from Germany to join extremists groups fighting in Syria and Iraq.