US Secretary of Defense Robert Gates said “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”.

WikiLeaks, a whistle-blowing group, posted online on Sunday some 91-thousand classified US military records from the past six years about the war in Afghanistan.

Robert Gates told reporters that the US had a "moral responsibility" to protect any individuals who might be in danger.

"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," he said.

Gates also said the military was reviewing its rules for safeguarding classified information.

The WikiLeaks material ranges from files documenting Afghan civilian deaths to evidence of US-Pakistani distrust.

Gates described the leak as a "mountain of raw data" that did not shed new light on US policy but could aid the enemy on the battlefield.