Well, the US is operating under 678 to enforce 688...

* Security Council Resolution 678 (1990), on the situation between Iraq and Kuwait (29 November 1990)
Authorizes Member States, unless Iraq on or before 15 January 1991 fully implements Security Council resolutions, to use all necessary means to uphold and implement resolution 660 (1990) and all subsequent relevant resolutions and to restore international peace and security in the area.

And so "necessary means" can be interpreted as no-fly zones by US, or not.
Technically it's illegal, technically it's not.

Cheers,
Tony
P.S. Larry, show me where Annan says "fire on US planes is OK by me".....