The documents, published online on July 26, give details of the Afghan war.

Mr Gates said they were potentially dangerous for US troops and allies, and an "aggressive investigation" would determine how the leaks occurred.

The Wikileaks website, which posted the documents, said they had been compiled by a variety of US units in 2004-09.

"The battlefield consequences of the release of these documents are potentially severe and dangerous for our troops, our allies and Afghan partners, and may well damage our relationships and reputation in that key part of the world," Mr Gates told reporters on July 29.

He said intelligence sources and methods, as well as military tactics could "become known to our adversaries".

Mr Gates added that he had called FBI director Robert Mueller on July 28 and "asked for the FBI's assistance in our investigation as a partner".

Wikileaks has said it tried hard to ensure that the release of the material "does not put innocents at harm", and held back about 15,000 reports.