US Central Command’s Twitter and YouTube accounts have been hacked by a group claiming to back Islamic State.
One message said: “American soldiers, we are coming, watch your back.”
It was signed by Isis, another name for the Islamic State. Some internal military documents also appeared on the Centcom Twitter feed.
US Centcom said it was taking “appropriate measures”. The Twitter account was later taken down so no tweets could be viewed.
The Centcom YouTube account was also hacked.
The hack happened as President Barack Obama was preparing a major speech on cybersecurity.
His spokesman Josh Earnest said the US is looking into the Centcom hacking. He said they were investigating the extent of the incident, and that there was a significant difference between a large data breach and the hacking of a Twitter account.
Some of the documents posted appeared to list names and phone numbers of members of the military as well as PowerPoint slides and maps.
An unnamed Pentagon official told Reuters the hacking was an embarrassment but did not appear to be a security threat.
And Professor Alan Woodward, from the University of Surrey, said he did not consider the attack to be a major breach of security.
“I wouldn’t say it’s trivial, but it’s just a slip,” he told the BBC.
“Twitter accounts are usually looked after by an individual in an organisation – it’s very easy to give away that password.
“In terms of if this is a hack into something secret, or sensitive – no, it’s not. An individual has made a slight mistake.”