The trusteeship agreement shall in each case include the terms under which the trust territory will be administered and designate the authority which will exercise the administration of the trust territory. 

Such authority, hereinafter called the administering authority, may be one or more states or the Organization itself.

High

Article 81, Chapter XII, Charter of the United Nations
http://www.un.org/en/sections/un-charter/chapter-xii/index.html