They are not "acting on the behalf of UN directives". They are requested and loaned to support specific action by member states that support the action. Member states often reject lending their troops to actions they don't like. If they were part of a "UN Army" no such rejection would be possible.Acting on behalf of UN directives, in violation of their enlistment agreements.
Bottom Line: THe UN can use military force to achieve it's directives. The fact that soldiers are ON LEASE is irrelevant.