US Central Command’s Twitter account appears to 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 told the BBC it was preparing a statement. The hack happened as President Barack Obama prepared a speech on cybersecurity.
Some of the documents posted appeared to list names and phone numbers of members of the military as well as PowerPoint slides and maps.
The Centcom YouTube account was also hacked.
American and coalition war planes have been undertaking air strikes against IS in Iraq and Syria.