Turkey's President Recep Tayyip Erdogan on Thursday accused Israel of "brutality" in boarding an aid flotilla that said it was sailing to help Palestinians suffering under the offensive in Gaza.

"This attack by the Israeli government against civilians sailing in international waters has once again shown the madness of its genocidal leaders trying hide their crimes against humanity in Gaza" and reveals "once again Israel's brutality", Erdogan said in a speech to his AKP party.

Erdogan's criticism came after the Istanbul chief prosecutor's office announced that it had opened an investigation into the arrest of Turkish citizens travelling with the flotilla.