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(CNN – Sept. 11, 2014) — A CIA assessment puts the number of ISIS fighters at up to 31,500, more than three times the number previously estimated, an agency spokesman told CNN on Thursday.

ISIS, the terror group that calls itself the Islamic State, “can muster between 20,000 and 31,500 fighters across Iraq and Syria,” the spokesman said.

Analysts and U.S. officials initially estimated there were as many as 10,000 fighters, including those who were freed from prisons by ISIS, and Sunni loyalists who have joined the fight as the group advanced across Iraq.

“This new total reflects an increase in members because of stronger recruitment since June following battlefield successes and the declaration of a caliphate, greater battlefield activity, and additional intelligence,” the spokesman said.

The news came a day after President Barack Obama laid out his plan to “dismantle and ultimately destroy” ISIS, including authorizing airstrikes.

The details about the increased number of fighters also came the same day news broke that the United States has begun surveillance flights over Syria, which a U.S. official told CNN is part of an effort to gather intelligence that will help the decision whether to launch airstrikes against ISIS .

The Pentagon is refining its targets based on improved intelligence-gathering, including the surveillance flights now under way over Syria, a Defense official said.

The United States “will take action at a time and place” of its choosing, a third official said. But while a broader campaign is not imminent, the officials — who all spoke on condition of anonymity — said the military can strike at any time if there is a target of opportunity.

The Pentagon spokesman suggested that striking ISIS also means going after the terror group’s leadership, something the United States has not done to this point.

“One of the ways you get at and you destroy the capabilities of an enemy like (ISIS) is to be pretty aggressive against them, and that does include disrupting their ability to command and control and to lead their own forces,” said Adm. John Kirby, the spokesman.

The President has been reviewing options and targets with his national security team, according to officials at the White House. Those officials would not characterize the number or scope of those targets but said the review was based on the expanded gathering of intelligence in Syria.

“These targets have been exposed because of the president’s early decision to ramp up our intelligence, surveillance and reconnaissance assets that were operating in the region,” White House spokesman Josh Earnest said. “By improving our visibility into the situation on the ground, Pentagon planners have been working with some success to develop additional targets.”