The Obama Administration has reason to believe some Americans who have fought alongside the Islamic State (ISIS) have returned to the United States, a senior Administration official confirmed on Monday.

In a briefing to discuss the administration's efforts to address the issue of so-called foreign terrorist fighters, the official said that the latest assessment from National Counterterrorism Center is that more than 100 Americans have attempted to fight in Iraq and Syria alongside ISIS, the al-Qaeda affiliated al-Nusra Front or other terrorist groups or their ilk.

"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, who spoke anonymously said, according to the New York Daily News. "These are FBI matters, I refer you to them on specifics."

It marked the first official government confirmation that at least some of the Americans fighting alongside the Islamist extremist group have returned to the U.S. On Monday morning, Secretary of State John Kerry said on MSNBC that "we have over 100 fighters there from America." He did not mention any of them had returned to the U.S., however.

Rep. Michele Bachmann (R-Minn) said on Newsmax last week that 2 people from her state had joined ISIS, and when she asked the FBI about it, she was told they would be allowed to return, and put on a watch list.

"I asked 'if they choose to return to Minnesota, will they be allowed to,'" She said. "'The FBI responded yes they will, they are American citizens."

Bachmann and others have called for these deluded fighters to be striped of their citizenship.

Other Obama Administration officials confirmed the FBI is watching suspected ISIS fighters who have returned.

"Well, with respect to the Americans who may be engaged in combat in Iraq and Syria, this is something that our national security agencies and counterterrorism team are taking very seriously," National Security Advisor Susan Rice said Friday. "It's something we track closely. And we are doing obviously all that we can to both gather the necessary information and take the appropriate precautions to the greatest extent that we possibly can."

President Barack Obama called a meeting of his National Security Council last Wednesday to "discuss the threat posed by foreign terrorist fighters to the U.S. homeland and nations around the world and the Administration's whole-of-government strategic response," the White House said. The fact the President is involved so heavily sends a clear message of resolve on the issue, according to officals.

The President will also chair a UN Security Council summit Wednesday on the subject, where the international body is expected to pass a resolution establishing an international legal framework for addressing the spread of foreign fighters.

The official at Monday's briefing said the NCTC believes that 15,000 fighters from 80 countries have tried to fight in Iraq and Syria, including 2,000 Europeans and the more than 100 Americans.