The number of Daesh fighters returning to Germany from Syria and Iraq is on the rise and more than 400 people are being watched, the head of German police said on Friday.

Holger Muench, president of the BKA federal police, told ARD television that the number of people leaving Germany for the two countries to fight with groups such as Daesh was dropping.

But there has been a simultaneous rise in the number of fighters returning home.

"The wave of departures is becoming flatter," he said. "In the meantime, we have more than 400 individuals who pose a threat and whom we must keep an eye on."

Muench said, however, that the suicide bombing in Istanbul this week in which 10 German tourists were killed was not a sign that the threat of a terrorist attack in Germany was higher than before.

The attack was carried out by a Daesh suicide bomber who entered Turkey as a refugee from Syria, the Turkish government said, adding that the militant had not specifically targeted Germans.

Germany and Turkey are both part of a U.S.-led military coalition that is carrying out air strikes against Daesh.