"The Department of Defence is not blocking any websites," Lieutenant Colonel Damien Pickart told AFP, in response to a Guardian report.
"We routinely take preventative measures to mitigate unauthorised disclosures of classified information onto DoD unclassified networks," he said.
"Should any website choose to post information the department deems classified, that particular content on the website will be filtered and remain inaccessible from DoD networks so long as it remains classified."
"The DoD is also not going to block websites from the American public in general," Pickart said, adding such a move would be a violation of civil liberties and privacy.
When anti-secrecy website WikiLeaks released a trove of US diplomatic cables and military documents about the wars in Iraq and Afghanistan in 2010, the Pentagon also used filters to block access to the information on military networks.