Article 3 Vienna Convention on the Law of Treaties

International agreements not within the scope of the present Convention

The fact that the present Convention does not apply to international agreements concluded between States and other subjects of international law or between such other subjects of international law, or to international agreements not in written form, shall not affect:

(a) the legal force of such agreements;

(b) the application to them of any of the rules set forth in the present Convention to which they would be subject under international law independently of the Convention;

(c) the application of the Convention to the relations of States as between themselves under international agreements to which other subjects of international law are also parties.

See also: Article 2 Vienna Convention on the Law of Treaties