The Twitter feed and the YouTube channel of U.S. Central Command were compromised on Monday, a Pentagon spokesman said.
"We are taking appropriate measures to address the matter. I have no further information to provide at this time," the spokesman said.
The hackers put up Islamic State propaganda and switched the avatar from the CentCom logo to a photo of a masked fighter.
Update at 5:30 p.m. ET: 'A Case Of Cybervandalism'
Saying that its "operational military networks were not compromised," CentCom has released a statement saying it is "viewing this purely as a case of cybervandalism."
The organization adds, "our initial assessment is that no classified information was posted and that none of the information posted came from CENTCOM's server or social media sites."
The FBI says it's working with CentCom to investigate.
Our original post continues:
On Twitter, the hackers released what they purported was a phone list of retired U.S. generals, as well as what appear to be presentation slides from the government-funded Lincoln Laboratory at MIT.
"Pentagon Networks Hacked," one tweet read.
At this time, however, there is no evidence Pentagon servers were compromised. The only thing we know for sure is that credentials for the social media channels, which are hosted on Twitter and YouTube servers, were compromised.
Quoting defense officials, Reuters reports that the images published on Twitter "do not appear to pose a security threat or include classified information."
Twitter and YouTube have suspended the accounts.