International law is part of United States law. Indeed, international law - or the "law of nations" in eighteenth century parlance - has been considered part of United States law since the founding. The Judiciary Act of 1789, the enabling legislation of Article III, establishes federal court jurisdiction over torts committed in violation of the law of nations.
Jinks, Derek P.
"The Federal Common Law Of Universal, Obligatory, And Definable Human Rights Norms,"
ILSA Journal of International & Comparative Law: Vol. 4
, Article 18.
Available at: http://nsuworks.nova.edu/ilsajournal/vol4/iss2/18