The document linked to below sets out the justification offered by the UN Security Council for its decision to create the International Criminal Tribunal for Yugoslavia under the powers set down in Chapter VII of the UN Charter.
The UN General Secretary at the time was Boutros Boutros Ghali. He had originally advised the Security Council that the UN Charter did not include powers to set up a criminal court. This could only be done via a unanimous vote of the General Council of the UN, or via a treaty agreed by the member states of the UN.
The members of the Security Council were eventually persuaded that, such was the scale and urgency of the humanitarian crisis in Yugoslavia, the powers of Chapter VII of the Charter could reasonably be extended to cover the creation of a criminal court. As we now know, the humanitarian crisis proved to be greatly exaggerated. It is also clear that the UN Charter intentionally and specifically excluded the power to set up a criminal court from the emergency powers set down in Chapter VII.
Boutros Boutros Ghali was apparently persuaded to revise his original position, but expressed clearly that the new Tribunal should apply only existing humanitarian law:
“In assigning to the tribunal the task of prosecuting persons responsible for serious violations of international humanitarian law, the Security Council would not, however, be creating nor purporting to “legislate” that law. Rather, the international tribunal would have the task of applying existing international humanitarian law.”
This was perhaps as far as the Secretary General dared to go. He was in any event vetoed by the USA for re-appointment to a second term of office – the only UN Secretary General not to be granted a second term.