The International Criminal Court's chief prosecutor has said the leaders of Isis could escape prosecution as most are from Syria and Iraq, both non-member states.
Fatou Bensouda said that the allegations of war crimes, genocide and crimes against humanity could be investigated if they are referred by the UN Security Council.
However the ICC's founding treaty limits the organisation's scope to crimes on the territory of or committed by citizens of member states.
"The jurisdictional basis for opening a preliminary examination into this situation is too narrow at this stage," Bensouda said in a statement.
Crimes committed by citizens of signatory states could still be investigated, with citizens from the UK, Australia, France, Germany, and Belgium having travelled to fight for the extremist Sunni organisation in Syria and Iraq.
However they could only be tried in signatory states, while the leadership cannot be prosecuted as citizens of non-member states.
"At this stage, the prospects of my office investigating and prosecuting those most responsible, within the leadership ... appear limited," the statement said.
Bensouda said that the group is reported to have carried out a litany of crimes.
"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 prosecutor said. "The commission of the crime of genocide has also been alleged."
The prosecutor added that non-members could grant the court special jurisdiction to investigate the crimes, but she could not influence that decision.
In March, the UN human rights body recommended the UN Security Council refer the group for investigation into war crimes and alleged attempted genocide of the Yazidis.
This week, mass graves where thousands of Shia cadets murdered by Isis are buried in Camp Speicher were uncovered, after Iraqi government forces retook Tirkit from the group.