US Defence Secretary Robert Gates said Thursday the Pentagon is conducting an aggressive investigation into the source behind the leaking of tens of thousands of classified documents about the war in Afghanistan to the internet whistle-blower WikiLeaks.

'This department is conducting a thorough, aggressive investigation to determine how this leak occurred, to identify the person or persons responsible, and to assess the content of the information compromised,' Gates told reporters at the Pentagon.

The 91,000 classified documents leaked to WikiLeaks emerged in press reports Sunday, prompting outrage from the White House and the Pentagon over worries they could expose sources and methods about operations in Afghanistan and put lives in danger.

The documents are largely old, covering the period from 2004 to 2009, and contain material that has already been in the public domain. But they also reveal details about the nature of military and intelligence operations.