US Central Command suspended its Twitter page on Monday after a group declaring sympathy for Islamic State jihadists hacked its social media accounts and posted internal documents.
In an embarrassing propaganda jab at the US military, a banner with the image of a hooded fighter and the words "CyberCaliphate" and "I love you ISIS" replaced CentCom's usual logo.
"We can confirm that the US Central Command Twitter and YouTube accounts were compromised earlier today. We are taking appropriate measures," a defense official told AFP.
The self-styled CyberCaliphate "is already here, we are in your PCs, in each military base," the hackers wrote on the seized Twitter feed before it was taken down.
Central Command, based in Tampa, Florida, oversees the US-led air war against the group in Iraq and Syria and other American operations in the Middle East and Horn of Africa.
And, in another embarrassing twist, the hacking of CentCom's social media accounts came as President Barack Obama was delivering a speech on cyber security.
It was unclear if the hacking represented a genuine threat to sensitive computer networks beyond the publicly accessible sites, and officials warned reporters not to jump to conclusions.
"It's a pretty significant difference between what is a large data breach and the hacking of a Twitter account," White House spokesman Josh Earnest told reporters.
"We're examining and investigating the extent of this incident."
US officials said it appeared no classified documents were posted by the hackers.
The hacked Twitter feed posted a phone directory of officers, which looked to be slightly out of date, as well what appeared to be a personal photo taken by troops handling a goat in an office.
Some power point slides related to North Korea and China, but they did not appear to contain classified information.
US commanders and senior officials have previously said the IS group has shown a great acumen for propaganda and for promoting itself to potential jihadist recruits.