WikiLeaks boss Julian Assange is a "political actor" determined to undermine international diplomacy by jeopardising efforts to resolve problems across the world, the US State Department said.

"He's not a journalist. He's not a whistleblower," State Department spokesman PJ Crowley said Thursday. "He is a political actor. He has a political agenda."

"He is trying to undermine the international system that enables us to cooperate and collaborate with other governments and to work in in multilateral settings and on a bilateral basis to help solve regional and international issues," Crowley added.

WikiLeaks is in the process of publishing more than 250,000 secret US diplomatic cables mostly focused on interactions with other countries over the last several years. The US has sharply condemned WikiLeaks, which describes it itself as dedicated to exposing government secrets, for releasing the documents.