Chapter 7 to the United Nations charter gives the Security Council (UNSC) the power to authorize states to use force in some particular case (for example, in the first gulf war the UNSC authorized the use of force against Iraq).
Now, normally any use of force by a state is subject to international humanitarian law (IHL – the laws of war). However, since the Security Council can authorize specific uses of force and not just blanket statements such as "force can be used," the question arises as to whether or not it can override IHL.
In theory, it would seem a little strange, particularly if we decide IHL counts as Jus Cogens (a peremptory norm of international law), but the charter explicitly states that Security Council decisions are binding on any state that is a member of the United Nations, and that any obligations a state has arising from the charter supersede other obligations under international law.
So, to make up a case for clarity, say the UNSC were to pass a resolution saying that because of the power of the Islamic State in Syria, not only is it legal for states to use force against them, but they no longer have to worry about killing civilians, since only by killing large numbers of people will the world be able to deter the terrorists. Would this decision be legal (and binding)?