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The Twitter and YouTube accounts of the US military command were suspended for a few hours after being hacked by a group claiming to back Islamic State.
One message on Centcom's Twitter feed 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.
Centcom said it was "cyber-vandalism" and not a serious data breach.
In a statement, it said there was no operational impact and no classified information was posted.
"We are viewing this purely as a case of cyber-vandalism," it said. Later on Monday, its Twitter feed became visible again, although not active.
Embarrassingly, the hack happened as President Barack Obama was giving a speech on cyber-security.
Reflecting on major breaches like a recent hack of Sony Pictures, Mr Obama said in his speech the US had been reminded of "enormous vulnerabilities for us as a nation and for our economy".
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President Obama said the internet creates "enormous vulnerabilities"
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.
Analysis - Jonathan Marcus, BBC defence correspondent This is an irritating hack rather than a matter of major security concern, but it will inevitably lead to a review to see if there are any more fundamental vulnerabilities in the US military's public facing web and Twitter accounts.
One message on Centcom's Twitter feed 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.
Centcom said it was "cyber-vandalism" and not a serious data breach.
In a statement, it said there was no operational impact and no classified information was posted.
"We are viewing this purely as a case of cyber-vandalism," it said. Later on Monday, its Twitter feed became visible again, although not active.
Embarrassingly, the hack happened as President Barack Obama was giving a speech on cyber-security.
Reflecting on major breaches like a recent hack of Sony Pictures, Mr Obama said in his speech the US had been reminded of "enormous vulnerabilities for us as a nation and for our economy".
Jump media player
Media player help
Out of media player. Press enter to return or tab to continue.
President Obama said the internet creates "enormous vulnerabilities"
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.
Analysis - Jonathan Marcus, BBC defence correspondent This is an irritating hack rather than a matter of major security concern, but it will inevitably lead to a review to see if there are any more fundamental vulnerabilities in the US military's public facing web and Twitter accounts.