A former Dutch soldier has been arrested on suspicion of involvement in the killing of combatants from the Islamic State group in Syria while fighting with a Kurdish militia.

Prosecutors say the suspect, whose identity was not released in line with Dutch privacy rules, fought with a Kurdish militia in Syria known as YPG, or People's Protection Units, which has become a main force fighting ISIS militants.

The 47-year-old man appeared Friday before an investigating judge in Rotterdam and was released on condition he turn in his passport, to prevent him from returning to Syria.

Prosecutors say killing ISIS fighters could lead to murder charges, but add that further investigation into the former soldier's involvement is required before a decision on any prosecution is made.