The US Marine Corps on Sunday urged "vigilance" after a group claiming to be Islamic State hackers published what they said were the names and addresses of 100 military personnel and urged supporters to kill them.
The warning came after a group calling itself the Islamic State Hacking Division posted information about members of the air force, army and navy, including photos and ranks, on the Internet, according to monitoring group SITE Intelligence.
The US Marine Corps said it was visiting all affected staff, and urged caution online.
"Vigilance and force protection considerations remain a priority for commanders and their personnel," US Marine Corps Lieutenant Colonel John Caldwell said in a statement.
"It is recommended Marines and family members check their online/social footprint, ensuring privacy settings are adjusted to limit the amount of available personal information."
Caldwell added the threat remained "unverified."
The self-proclaimed IS Hacking Division said it took the information from government servers, databases and emails and called on its followers to mount attacks.
The group said the 100 military staff had targeted the Islamic State group in Iraq, Syria and Yemen.
"With the huge amount of data we have from various different servers and databases, we have decided to leak 100 addresses so that our brothers residing in America can deal with you," the group wrote.
"Now we have made it easy for you by giving you addresses, all you need to do is take the final step, so what are you waiting for?"
A defense source, however, told The New York Times most of the information was in fact available in public records and did not appear to have been hacked from government servers.
- 'CyberCaliphate' -
A defense department official told AFP they were investigating the posting.
"I can't confirm the validity of the information, but we are looking into it," the official said.
"The safety of our service members is always a concern."
The United States is leading an international coalition targeting the Islamic State group in Iraq and Syria, where the Islamist militants have taken over swaths of territory.
IS has claimed responsibility for hacking attacks in the United States in the past, including against Central Command, which oversees the air war against the IS group.
In January, the hacked Centcom Twitter feed posted what appeared to be an office phone directory of officers that was slightly out of date, forcing the military temporarily to take down the command's Twitter feed.
The words "CyberCaliphate" and "I love you ISIS" replaced Central Command's usual logo on Twitter and YouTube before the pages were suspended.
This month, the FBI said it was investigating a string of hacks that defaced US websites with imagery from the Islamic State jihadist group.
Websites including a racecar speedway in the state of Ohio, a used-clothes store in Missouri and a church in Canada were defaced with the radical group's black flag.
Attacks against similar low-profile websites were also recorded in Montana, New York, Massachusetts and Minnesota.
The Islamic state group has relied on an aggressive propaganda campaign online to appeal to possible recruits to join and support the violent Islamists in Iraq and Syria.
US commanders and senior officials have previously said the IS group has shown a great acumen for propaganda and for promoting itself to potential jihadist recruits.
The group has called for so-called lone wolf, or freelance, attacks in coalition countries, including the United States, Canada, Japan and Italy.