The chief prosecutor of the International Criminal Court (ICC), Fatou Bensouda, said in a statement Wednesday that there was little hope "Islamic State" leaders would ever face the tribunal.

Crimes of "unspeakable cruelty," including mass executions, sexual slavery, torture, rape and mutilation have been reported, with acts of genocide also alleged, Bensouda said.

Iraq and Syria, where "IS" is predominantly located, are not signatories to the ICC's founding Rome Statute which grants the court jurisdiction to hear criminal cases.

"The jurisdictional basis for opening a preliminary examination into this situation is too narrow at this stage," Bensouda said.

However, Bensouda's office has been reviewing whether it has powers to exercise "personal jurisdiction" over foreign IS fighters who are nationals of ICC member states, including those from Tunisia, Jordan, Britain, France, Germany, Belgium, the Netherlands and Australia, she added.

"Some of these individuals (from member states) may have been involved in the commission of crimes against humanity and war crimes," the statement read.

Many nationals of ICC member states are suspected of committing atrocities while fighting for IS, including British citizen "Jihadi John," who is thought to be responsible for a number of hostage beheadings.

As happened with Libya in 2011, the United Nations Security Council could refer the situation in Iraq and Syria to the international tribunal, she said, adding that countries with nationals who are fighting with IS also have the option to launch their own legal proceedings.

jlw/kms (AFP, Reuters, AP)