Concerns that the Islamic State group could pose a threat to the American homeland are rising. Some Americans who went overseas to fight with ISIS have returned home, the Obama administration announced on Monday. This marks the first time the U.S. government has confirmed that Americans involved with the militant group are on American soil.

Administration officials cited figures from the National Counterterrorism Center that estimate more than 100 Americans attempted to join forces with ISIS militants in Iraq and Syria, though exact numbers are unavailable.

“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,” a senior administration official said. “These are FBI matters, I refer you to them on specifics.”

There was no further information on how many of these Americans might be back home or whether they have been detained.

U.S. Secretary of State John Kerry said on MSNBC on Monday that ISIS has “100 fighters there from America,” though he didn't discuss whether any had returned.

Earlier this month, President Obama said that there was no evidence of plots against the U.S. by the Islamic State group, but that intelligence officials were investigating the issue.

“Our intelligence community believes that thousands of foreigners -- including Europeans and some Americans -- have joined them in Syria and Iraq,” Obama said. “Trained and battle-hardened, these fighters could try to return to their home countries and carry out deadly attacks.”

President Obama is expected to chair a Security Council meeting this week, during which the United Nations will seek a resolution that calls on countries to strengthen the laws and programs that could prevent foreign citizens from joining terrorist organizations in the Middle East.

Later this fall, the White House is expected to host a summit on how to combat violent extremism, the Hill reported.

Meanwhile, the New York Daily News wrote that the NYPD is already planning for threats to New York City by terrorists affiliated with the Islamic State group. The department’s terror chief, John Miller, said: “We have a small number of investigations in progress in coordination with the FBI's Joint Terrorist Task Force. There is a small number of people we're looking at in New York. Beyond that, if evidence is developed that a person worked for, or fought for a designated terrorist organization, they will be prosecuted."