Germany's domestic intelligence chief is urging Turkey to do more to prevent extremists crossing into Syria to join the Islamic State group and other terrorist organizations.

Authorities say that at least 550 people from Germany have traveled to Syria and Iraq to join extremist groups, along with more from several other European countries. Hans-Georg Maassen, the head of Germany's domestic intelligence agency, says Turkey is a "key country" because well over 90 percent of the radicals traveled via Turkey.

Maassen told ARD television Monday that while efforts to stop extremists crossing into Syria are partially successful, the number that have arrived remains too high — "so it is all the more necessary that the Turks take further measures."

Turkish Prime Minister Ahmet Davutoglu is visiting Berlin on Monday.