The US administration calculates that 2,000 Europeans are among the 15,000 foreign combatants in IS ranks, Efe quoted an official as saying.
The figure of 100 Americans "includes those who have gone, those who have tried to go, some who have come back and who are under active - the FBI is looking at them", said the source, according to TIME magazine's online edition.
2,000 Europeans are among the 15,000 foreign combatants in IS ranks
The United States and Europe are concerned about the possibility that those foreign combatants are using their passports -- once they return to their countries of origin -- to travel and organise attacks in the West.
On Wednesday, President Barack Obama will head a special meeting of the heads of state and government at the UN Security Council that will focus on the threat posed by those individuals.
The United States has already circulated, with an eye toward that gathering, a draft resolution including measures designed to contain the flow of foreign combatants and the White House forecasts that it will be unanimously.