The new estimate was provided by the Pentagon in a clarification of comments made Wednesday by Defense Secretary Chuck Hagel during a CNN interview at the Naval War College in Newport, Rhode Island.
During the interview Hagel said, "We are aware of over 100 U.S. citizens who have U.S. passports who are fighting in the Middle East with ISIL forces." He added, "There may be more. We don't know." ISIL is another name used to refer to the group.
On Thursday, a Pentagon spokesman clarified that the number used by Hagel was actually a reference to the more than 100 Americans the U.S. believes have traveled to Syria to join various groups. That estimate has been discussed publicly by U.S. officials for several months.
Col. Steve Warren told reporters that "We believe that there are approximately 100 American passport holders operating inside of Syria, we don't know specifically who they are aligning themselves with. We believe that there are maybe a dozen that are with ISIL."
Until now intelligence officials have only said that "a small handful" of the approximately 100 Americans fighting in Syria had been fighting with ISIS.
On Tuesday Matt Olsen, the director of the National Counterterrorism Center, said at a Washington thinktank that more than 12,000 foreign fighters have gone to Syria over the past three years to join rebel fighting groups including an estimated 1,000 Europeans "and more than 100 Americans."
Olsen said that "many of these foreign fighters have joined ISIL's ranks, and the group may use these fighters to conduct external attacks." But Olsen also said the militant group lacks the capacity to launch large-scale terror attacks on U.S. soil like al Qaeda.
According to Olsen the FBI has arrested more than half a dozen individuals who have sought to travel from the U.S. to Syria to support ISIS.
The U.S. believes that the group has grown in size to 10,000 fighters who are operating on both sides of the Syria-Iraq border.
Last week the White House confirmed that Douglas McAuthur McCain, an American citizen from Minnesota, had been killed in Syria while fighting with ISIS forces. Reports that another American citizen had been killed in the same incident could not be confirmed by U.S. officials.
In May, the State Department confirmed that an American citizen was involved in a suicide bombing attack in Syria. Moner Mohammad Abu-Salha, 22, from Florida, was affiliated with Jabhat al-Nusra, one of several Islamic rebel groups fighting to overthrow the regime of Syrian President Bashar al Assad.
A U.S. official told ABC News that it is easier for foreign fighters to join the ISIS ranks than it is to join al Nusra.
The official said it appears that al Nusra seems to have a much stricter vetting process for any foreign fighters seeking to join their ranks than does ISIS.