Wikileaks's decision to post on its website more than 92,000 classified documents related to war in Afghanistan is a breach of federal law and an investigation is on the leak of secretive Pentagon documents, the White House said today.

"They constitute a potentially national security concern," White House spokesman Robert Gibbs today said as he informed reporters that he and others in the Obama Administration were informed about the upcoming release of such a document by the news outlets that were given pre-release access to these documents by Wikileaks.

"I remember talking to the (US) President sometime last week after discussions with news organisations that these stories were coming," he said in response to a question.

"I think our reaction to this type of material, a breach of federal law, is always the same, and that is, whenever you have the potential for names and for operations and for programs to be out there in the public domain, that it, besides being against the law, has the potential to be very harmful to those that are in our military, those that are cooperating with our military, and those that are working to keep us safe," Gibbs said.

There is an ongoing investigation into the leak of classified documents, he said, but refrained from going into details.

"I think there is no doubt that this is a concerning development in operational security. It poses a very real and potential threat to those that are working hard every day to keep us safe," Gibbs said.

Noting that these documents being made public harm national security, Gibbs said it's not the content as much as it is there are names, there are operations, there's logistics, there's sources.

"All of that information out in a public way has the potential to do harm. If somebody is cooperating with the federal government and their name is listed in an action report, I don't think it's a stretch to believe that that could potentially put a group or an individual at great personal risk," he argued.

Gibbs said they were shown the documents by Wikileaks.

"Nobody in this government was afforded the opportunity to see what they do or don't have. I don't know that this question is relevant for me as much as it is for him," he said, adding the White House did notify the relevant committees on Capitol Hill that these documents were about to go online.

The spokesperson said through The New York Times, he passed a message to the head of Wikileaks that that could harm personnel or threaten operations or security.