The United States has reportedly briefed Australia, Britain and Canada ahead of the expected release of classified US documents.

The whistleblower website WikiLeaks claims it will be posting 2.8 million secret documents, which it says will redefine global history.

The documents are said to include sensitive cables from the US State Department to American ambassadors around the world.

Canada's defence minister, Peter MacKay, says he is concerned about what may be coming.

"If there's anything in there that endangers soldiers or speaks about operational detail, then I am worried," he said.

"We've seen from the last round that that kind of commentary can be very dangerous for soldiers serving in Afghanistan."