Update as of 12.30 a.m. EST: The official Twitter account of the U.S. Central Command (CENTCOM), which was suspended on Monday following a hacking attack by a pro-Islamic State group named Cyber Caliphate, is back online.

CENTCOM said, in a statement released late Monday, that its military networks had not been compromised, and that the act of “cybervandalism” had not had any “operational impact.” However, the official YouTube page of CENTCOM, which was also hacked, has been terminated.

UPDATE 5:45 p.m. EDT: The FBI Monday said ti would investigate the hacking of the Twitter and YouTube accounts of the U.S. Central Command -- CENTCOM. Reuters said the FBI would work with the Defense Department to determine "the nature and scope" of the incident. Hackers who claimed responsibility for the cyberattack said they were sympathetic to the Islamic State group.

CENTCOM said the hack did not compromise its military networks and had no operational impact. The accounts were compromised for about 30 minutes but no classified information was posted, CENTCOM said.

In an ironic twist, the National Counterintelligence Security Center sent its first tweet just hours before the hack. The inaugural tweet said the agency had "said too much already." The center was formed last year to organize U.S. intelligence efforts.

Original post:

A U.S. military Twitter account was suspended Monday after apparently being hacked by a group calling itself the Cyber Caliphate, whose members are supposedly loyal to the Islamic State group. The account belonged to U.S. Central Command, also called Centcom. A YouTube page also appeared to have been breached.

Hackers posted tweets threatening American troops as well as retired high-ranking U.S. officers. Some of the tweets included screenshots of what looked like spreadsheets containing the personal contact information, including home addresses, of several Army generals. A Defense Department official told NBC News that the Centcom Twitter account had “clearly been hacked.”

The first tweet appeared around 12:30 p.m. EST and read, “Pentagon networks hacked AMERICAN SOLDIERS WE ARE COMING, WATCH YOUR BACK. ISIS. #CyberCaliphate.” The Twitter profile’s image was replaced with a photo including the words, “i love you isis [sic].”

A link in one of the tweets included a memo, which read, “In the name of Allah, the Most Gracious, the Most Merciful, the CyberCaliphate under the auspices of ISIS continues its CyberJihad. While the US and its satellites kill our brothers in Syria, Iraq and Afghanistan we broke into your networks and personal devices and know everything about you.”

A follow-up tweet stated, “ISIS is already here, we are in your PCS, in each military base.”

The White House played down the supposed hack. “There is a significant difference between what is a large data breach and the hacking of a Twitter account," White House press secretary Josh Earnest told reporters Monday about an hour after the first tweet appeared. Centcom frequently tweets out updates on U.S. airstrikes against ISIS, a militant Islamist group that has taken control of parts of Iraq and Syria.

Members of Cyber Caliphate last week broke into the Twitter accounts of at least two U.S. news outlets in New Mexico and Maryland, according to My San Antonio. Images alleging to show “confidential data” from local residents’ computers were tweeted out from accounts belonging to the Albuquerque Journal and a Maryland TV station called WBOC. The companies’ banner images were changed to black-and-white images with what appeared to be the ISIS flag.