Hackers claiming to be working on behalf of Islamic State militants briefly seized control of the Twitter and YouTube sites of the military's US Central Command.
The Pentagon swiftly suspended the sites and said no classified material was breached.
The Twitter site was filled with threats that said "American soldiers, we are coming, watch your back." Other postings appeared to list names, phone numbers and personal email addresses of military personnel as well as PowerPoint slides and maps.
Most of the material was labeled "FOUO," which means "For Official Use Only," but none of it appeared to be classified or sensitive information, suggesting the hackers did not breach classified material.
One of the documents appeared to be slides developed by the Massachusetts Institute of Technology's Lincoln Laboratory, a federally funded research and development centre focused on national security. The slides appeared to depict what it called "scenarios" for conflict with North Korea and China.
The tweets came shortly after US Central Command posted its own tweets about the US and partner nations continuing to attack Islamic State terrorists in Iraq and Syria and one repeating a report that said France will deploy an aircraft carrier to the fight.
The hackers titled the Twitter page CyberCaliphate with an underline that said "i love you isis." And the broader message referred to the ongoing airstrikes against the Islamic State group in Iraq and Syria and threatened, "We broke into your networks and personal devices and know everything about you. You'll see no mercy infidels. ISIS is already here, we are in your PCs, in each military base."
It added: "US soldiers! We're watching you!"
The intrusion on the military Twitter account carried the same logo, CyberCaliphate name and photo that appeared on the Albuquerque Journal's website in late December when one of its stories was hacked.
Some IS militant videos also were posted on the YouTube site, purporting to show military operations and explosions.
"This is something we're obviously looking into, and something we take seriously," White House spokesman Josh Earnest said.
But he cautioned against comparisons to the broader hack attack against Sony. "There's a pretty significant difference between what is a large data breach and the hacking of a Twitter account," he said.
A senior defence official confirmed that the two accounts were compromised and said US Central Command was taking appropriate measures to address the matter.
The military suspended the Central Command Twitter site and terminated the YouTube site. This is not the first time that US government websites or other accounts have been hacked. It was not clear whether the site was attacked by the insurgent group or by sympathisers.