"At least from my perspective, it has only been very recently that I was aware of the magnitude of the number of documents that had been leaked," Mr Gates said at a joint Pentagon news conference with Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff.
"The reality is, at this point, we don't know how many more there are out there. It could be a substantial additional number of documents. And we have no idea what their content is, either," Gates added.
The whistleblowers' website, WikiLeaks, posted a trove of secret documents on Sunday, Jul 25 evening raising several questions on the military operations in Afghanistan.
The document uncovered the details on thousands of civilians killed by coalition forces and in Taliban strikes. Besides this, it also revealed the fears of NATO that Pakistan and Iran were fueling insurgency.
Following the leak, the founder of the website, Julian Assange, had indicated that there was more to come.
Referring to this, Gates said, "My impression is that the head of WikiLeaks has acknowledged that he has thousands of additional documents that he has not yet posted. So we have his own statement to that effect."
To a question if there has been any attempt to reach out to WikiLeaks, the defence secretary answered, "Not that I'm aware of (having any kind of dialogue with WikiLeaks). I'm not sure why we would. Do you think he's going to tell us the truth?"
And to a question if Assange faces criminal prosecution, Gates said, "I have no idea. The investigation should go wherever it needs to go."