Islamic State (Isis) has committed “crimes of unspeakable cruelty” in Syria and Iraq but the international criminal court (ICC) does not have jurisdiction to open an inquiry, its prosecutor has said.

Fatou Bensouda said in a statement on Wednesday that the court was unable to prosecute since neither Syria nor Iraq was a member of the court and the United Nations security council has not asked for an investigation.

“The jurisdictional basis for opening a preliminary examination into this situation is too narrow at this stage,” the statement said.

The Rome statute (pdf), the ICC’s founding treaty, is open to participation by states. The prosecutor can only investigate and prosecute crimes committed on the territory or by the nationals of states that have joined the ICC statute or which have otherwise accepted the jurisdiction of the ICC through an ad hoc declaration to that effect.

Bensouda said even though she had information that “several thousand foreign fighters”, including citizens of countries that are members of the court, had joined Isis and “may have been involved in the commission of crimes against humanity and war crimes”, the group appeared to be led by Syrians and Iraqis. “Thus, at this stage, the prospects of my office investigating and prosecuting those most responsible, within the leadership ... appear limited,” the statement said.

The Gambian jurist said she made her comments after repeatedly being asked whether she could prosecute over Isis atrocities.

“Crimes of unspeakable cruelty have been reported, such as mass executions, sexual slavery, rape and other forms of sexual and gender-based violence, torture, mutilation, enlistment and forced recruitment of children and the persecution of ethnic and religious minorities, not to mention the wanton destruction of cultural property,” the statement said. “The commission of the crime of genocide has also been alleged.”

She added that nations that were not members of the court, or the security council, could grant her jurisdiction, but she could not influence that decision.

