ISIS terrorist in Raqqa, Syria (file)
ISIS terrorist in Raqqa, Syria (file)Reuters

US authorities are highly concerned that Islamic State (ISIS) is targeting its military servicemen - not only in the Middle East, but at home. 

An internal memo obtained by ABC News Monday night revealed that the Federal Bureau of Investigations (FBI) and Department of Homeland Security (DHS) have asked veterans to carefully monitor their social media accounts in the threat's wake. 

"The FBI and DHS recommend that current and former members of the military review their online social media accounts for any information that might serve to attract the attention of ISIL [ISIS] and its supporters," the federal bulletin sent to law enforcement agencies Sunday said, advising that troops "routinely exercise operational security in their interactions online."

"The FBI recently received reporting indicating individuals overseas are spotting and assessing like-minded individuals who are willing and capable of conducting attacks against current and former U.S.-based members of the United States military," it added.

The primary concern is over "copycat attacks" similar to the October attack on the Canadian parliament. In that attack, the gunman, 32 year-old Michael Zehaf-Bibeau, opened fire at Canada's National War Memorial and Parliament Hill, killing army reservist Cpl. Nathan Cirillo. Zehaf-Bibeau was then shot and killed by security.

Ottawa later stated that Zehaf-Bibeau was driven by "ideological" motivations - a reference to the Canadian citizen's aspirations to join ISIS and had ties to jihadists in Canada who shared his Islamist views. 

ISIS has threatened attacks in North America on more than one occasion, and has specifically addressed the videoed executions of American journalists as "messages to [US President Barack] Obama" over the US-led Western air campaign against the group in Iraq and Syria. 

But the bulletin belies the brave face put on by US Secretary of State John Kerry regarding the ISIS threat, as Kerry stated just weeks ago that Washington is "not intimidated" by the Islamists. 

"The United States does not go in search of enemies in the Middle East. There are times, however, and this is one, when enemies come in search of us," Kerry said in November. 

ISIS "leaders assume that the world will be too intimidated to oppose them," he added. "But let us be clear: We are not intimidated."