WikiLeaks on Thursday confirmed reports that it has lost control of a cache of U.S. diplomatic cables that it has been publishing in recent months, saying a security breach has led to the public disclosure of hundreds of thousands of the unredacted documents.

The website quickly sought to deflect blame for the leak of the leaked classified cables. It accused the U.K. newspaper the Guardian—which last year was WikiLeaks' partner in publishing some of the cables—of publishing in a book a password that unlocks an encrypted file...