Some 40,000 foreign terrorists have joined the ISIS group in Iraq and Syria from at least 120 countries in Europe, Africa and southeast Asia, General Michael Nagata said at Washington's Center for Strategic and International Studies think-tank.
"We know we have killed several thousands of foreign fighters in Iraq and Syria," said Nagata, director of the National Counterterrorism Center's Directorate for Strategic Operational Planning.
a"But we are unable to give you a precise number. It's a substantial number."
"We really don't know" how many remain despite the massive resources trying to determine the number, he added.
Estimating the security threat they pose their home countries on their return is also problematic, he said.
"ISIS and the foreign terrorist fighter problem is not a monolith," he said, using an alternate acronym for the IS group. "It's an incredibly diverse set of actors with an incredibly diverse set of motivations."
The ISIS group has released videos apparently showing children executing prisoners in order to "create a propaganda-driven impression that all the children of ISIS fighters will take up the black flag, that they are ready now to commit acts of violence even if they are only 11 or 12," Nagata said.
Although "there may be some truth" in that perception, he said, "it is probably not as strong or widespread as what the Islamic State group wants us to believe."
The ISIS group has lost much of the territory it once held during a more than two-and-a-half-year military campaign by a US-led international coalition.
Coalition-backed Iraqi forces are currently battling to recapture the northern city of Mosul from the jihadists.
US-backed fighters of the Syrian Democratic Forces are also mounting an offensive to retake the Syrian city of Raqa, the ISIS group's last major stronghold in that country.