With an increasing number of Westerners traveling to Iraq and Syria to join the Islamic State, the Federal Bureau of Investigation is currently tracking close to 150 Americans who might have recently traveled to Syria in an attempt to battle alongside foreign jihadist groups, FBI Director James Comey said on Tuesday.

"We have tracked coming up on close to 150 people who traveled from the United States to Syria, for all manner of motivations. A significant number of them to fight," he told reporters at a briefing in Boston.

While monitoring the movement of these suspects is a priority, the FBI's top concern has been the possibility of these Americans returning back to the United States armed with the training, expertise and connections required to launch an attack on home soil, Reuters reported.

"We are determined not to allow future lines to be drawn from a terrorist diaspora out of Syria to a future 9/11," he said.

Specifically, the FBI is focusing on suspected Americans believed to have joined forces with the militant group Islamic State, which has captured large swaths of land in Syria and Iraq, Comey said, adding that the terrorist organization's sophisticated online recruitment campaign has been its most resourceful tool, with ISIS continuously taking to social media to publicize the beheadings of Western hostages.

"There we see somewhere in excess of a dozen (Americans who have joined Islamic State) that we have a pretty good handle on," Comey said. "I don't have high confidence that I see the entire universe," he added.

Although Comey refused to divulge details about how many suspected fighters were believed to have returned to the U.S. after having been closely associated with foreign jihadist groups for months, he did state that the FBI was tracking their movements "very closely," The Washington Times reported.

"That is something that we track very closely," he said. "I don't want them to know too much about what we know."

Meanwhile, a senior Kurdish leader revealed that at least 200,000 Islamic State militants are reportedly fighting in Iraq and Syria against the international coalition, far larger than what has previously been estimated by the CIA.

While intelligence officials had reported the terrorist organization as having up to 31,500 fighters, it is now being speculated that ISIS has recruited an army hundreds of thousands strong, further proven by their ability to simultaneously cover and attack multiple widely separated fronts in Iraq and Syria, The Independent reported.

"I am talking about hundreds of thousands of fighters because they are able to mobilize Arab young men in the territory they have taken," Fuad Hussein, the chief of staff of the Kurdish President Massoud Barzani, told The Independent in an exclusive interview on Sunday.