The Pentagon building

The US Pentagon admitted that around 100 American nationals are fighting alongside the terrorist groups operating inside Syria, with a dozen of those are believed to have joined the Islamic State (IS) terrorist organization, formerly known as the Islamic State of Iraq and Syria (ISIS) or (ISIL).
Spokesman for the Pentagon Colonel Steven Warren said there are around 100 US nationals “operating inside of Syria,” noting that it is unclear to which armed groups they are linked.
“We believe there are maybe a dozen that are with ISIL (IS militants),” he added.
NBC News earlier revealed that investigations are underway in the US to probe into the issue of American nationals going to Syria and joining ISIS ranks there.