Twelve more minutes of research:

""...a Security Council referral ... to the ICC would be the only reasonable option for prosecuting members of ISIL ...  However, this avenue for investigating and prosecuting the highest ranking members of ISIL is also problematic in view of the composition of the Security Council. ... the United States may be concerned that a potential investigation by the ICC would also scrutinize its own military operation. So the United States and indeed other members of the Council could advocate for the inclusion of an exclusion of jurisdiction clause for their own nationals as was the case with previous referral resolutions on Darfur and Libya. Nevertheless, a resolution would in all likelihood fail ... as happened last year when a resolution to refer the situation in Syria to the ICC failed ... . "

https://www.asil.org/insights/volume/19/issue/21/prosecuting-isil-international-criminal-court-challenges-and-obstacles 
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