FRANKFURT (AFP) - Hundreds of Germans have left their home country to fight alongside extremists in Syria and Iraq, Interior Minister Thomas de Maiziere said.

"We estimate 550. Just a few days ago we had 450," the minister told German television channel Phoenix on Friday.

"These young people... were radicalised in Germany, within this society. That's why prevention must be accompanied by repression," he added.

Most of those who have joined the Islamic State in Iraq and Syria's (ISIS) cause are men, although some women have also travelled to the two war-torn countries.

De Maiziere said authorities are keeping a close watch on some 230 more people who are considered potential threats on German soil.

"We cannot exclude, and in certain cases it's actually quite possible, that they are preparing an attack," the minister said.

In mid-October, Germany announced new measures to prevent its citizens from travelling to join ISIS, including confiscating their identity papers.

Concerns are mounting in Europe over the growing national security threat posed by militants returning from war-ravaged Syria and Iraq.