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The White House believes some Americans who have fought for Islamic State in Iraq and Syria (IS or ISIS) and other groups have returned to the US, an unnamed Obama administration official admitted for the first time Monday.

Over 100 Americans are believed to have joined Islamist extremists, the official admitted, during a press briefing on foreign nationals fighting in the Syrian Civil War.

Several watchdog groups and intelligence agencies have released estimates regarding Americans involved in the fighting, but Monday marks the first admission that US-born jihadists have returned home from a White House official.

“It includes those who’ve gone, those who’ve tried to go, some who’ve come back and are under active—the FBI is looking at them,” the official said. “These are FBI matters, I refer you to them on specifics.”

“This is worked on a case by case basis,” the official added, saying the White House “defer[s] to the FBI” on further questions. Specific organizations known to have recruited Americans, he said, are IS, Al Qaeda, and the Al-Nusra Front.

Hours earlier, US Secretary of State John Kerry also cited "100 Americans in Syria" in an MSNBC interview, but declined to mention any who have returned. 

Exact data on foreign nationals globally is uncertain. Some estimates claim that up to 75,000 foreign nationals may now have joined Syrian rebel groups.

But the Obama administration official added Monday that the National Counterterrorism Center (NCTC) has placed the official estimate at closer to 15,000 foreign nationals from 80 countries, with roughly 2,000 Europeans and over 100 Americans. 

Outrage and concern over American involvement with IS has risen dramatically over the past month, after the cruel beheading murders of three US and British journalists and escalating threats from the terror organization. 

News of returning terrorists with US citizenship surfaces after two US nationals made headlines for attempting to help IS, with one Colorado woman aiming to marry into the group and a New York man actively involved in sending money and providing training for the organization.