Pro-Syrian regime hackers posted messages on a US Marine Corps recruiting website on Monday, urging troops to defy orders from President Barack Obama.
The hackers showed photos of people in American uniforms holding hand-written signs saying they would not fight for Al-Qaeda in Syria.
"Obama is a traitor who wants to put your lives in danger to rescue al-Qaeda insurgents," the message read, according to a screenshot from The Wall Street Journal.
The US Marine Corps confirmed the intrusion but said the affected site, Marines.com, the official recruitment portal for the Corps, was back to normal.
"Marines.com itself was not compromised or 'hacked," said Captain Eric Flanagan in a statement. "It was redirected for a limited amount of hours overnight."
The site "is now operating normally and our team is actively monitoring the situation and prepared to mitigate any future issues," he added.
The Marine Corps could not confirm who had defaced the site but the Syria Electronic Army reportedly claimed responsibility.
The group has claimed credit for recent cyber hacking of The New York Times, The Washington Post and other websites.
The Marine Corps site was hacked after President Obama said he backed military action against the Syrian regime in retaliation over the alleged use of chemical weapons against its own people.
Obama has put off any strike for the moment, saying he first wanted Congress to weigh in on the issue.
The Syrian regime has portrayed the country's civil war as a fight against "terrorists" while US officials have acknowledged the presence of Al-Qaeda militants among a divided opposition.
Explore further: New York Times website still down after hack attack (Update)