Story highlights U.S. aircraft are conducting 60 reconnaissance flights a day in Iraq, official says

More than 15,000 fighters, including 2,000 Westerners, have gone to Syria, CIA source says

CIA increases its assessment of number of ISIS fighters to up to 31,500

Air campaign is not imminent, but military can strike target of opportunity, officials say

A CIA assessment puts the number of ISIS fighters at possibly more than three times the previous estimates.

The terror group that calls itself the Islamic State "can muster between 20,000 and 31,500 fighters across Iraq and Syria," a CIA spokesman told CNN on Thursday.

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.

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Mass executions and videotaped beheadings, including those of two American journalists, have led to the push for a broader counterterrorism mission, including possible airstrikes in civil war-torn Syria.

It's unclear how the ISIS ranks swelled, and whether the increased numbers include recruits from within Iraq.

More than 15,000 foreign fighters, including 2,000 Westerners, have gone to Syria, a CIA source told CNN on Thursday. The fighters come from more than 80 countries, the source said.

It's not immediately clear whether these were primarily ISIS fighters or were dispersed among a number of groups fighting the Syrian government.

The details about the increased number of fighters followed news 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 Rear Adm. John Kirby, the spokesman.

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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."

While officials work to develop targets in Syria, the U.S. military is flying about 60 surveillance and reconnaissance flights a day over Iraq, according to the U.S. official.

Those flights have "developed over time a tremendous clarity of the intelligence picture within Iraq" and has offered a better understanding of the targets, the official said.

The Pentagon also plans to have "armed and manned" intelligence, surveillance and reconnaissance aircraft flying from Irbil, the Kurdish regional capital, in the coming days to supplement similar unmanned aircraft carrying out missions over Iraq.