The Pentagon says WikiLeaks may have more secret government documents, beyond those the whistle-blower website has released or mentioned publicly, a spokesman said.
"We have reason to believe they have other documents as well," Colonel Dave Lapan told reporters, without elaborating.
WikiLeaks, which portrays itself as a whistle-blowing media outlet, released last week an unprecedented 400,000 classified US documents on the Iraq war, and in July posted 77,000 secret US files on the Afghan conflict.
WikiLeaks has said it still has 15,000 documents and video on Afghanistan in its possession.
Media reports have also speculated that the website has tens of thousands US State Department cables, but WikiLeaks has denied this.
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WikiLeaks and the Pentagon have been engaged in a war of words over the website's dump of secret military files, with US officials accusing the organisation of endangering the lives of troops and civilians who worked with US-led forces.
WikiLeaks argues the release of the documents has shed light on the wars, including alleged widespread torture by Iraqi forces and reports that suggested 15,000 additional civilian deaths in the Iraq conflict.
Although US officials condemned WikiLeaks as jeopardising the security of its troops, Australia said Tuesday that the release of documents in July had not hampered its operations in Afghanistan.
"The investigation found that the leaked documents have not had a direct significant adverse impact on Australia's national interests," the Australian Defence Force said in a statement.
While many reports published by WikiLeaks would not routinely be made public for security reasons, much of the data had already been made public by the military, in many cases in greater detail, the Australian report said.
WikiLeaks has not revealed the source of the secret documents but suspicion has fallen on Bradley Manning, a US Army intelligence analyst arrested in May after the website released footage of a US Apache helicopter strike in Iraq in which civilians died.
Manning is being held at the Marine base in Quantico, Virginia south of Washington.
