ISIS can ''muster'' between 20,000 and 31,500 fighters, CIA says

ISIS can ''muster'' between 20,000 and 31,500 fighters, CIA says

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STORY HIGHLIGHTS

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

Pentagon is refining its targets based on improved intelligence-gathering, an official says

Part of intelligence-gathering effort is surveillance flights over Syria, the official says

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

(CNN) -- [Breaking news update 6:37 p.m. ET]

ISIS, 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.

That is two to three times more than the 10,000 fighters previously estimated by the CIA.

"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.

[Original story published at 6:27 p.m.)

U.S. surveillance flights under way over Syria, official says

The United States has begun surveillance flights over Syria, part of an effort to gather intelligence that will help the decision whether to launch airstrikes against ISIS, a U.S. official told CNN on Thursday.

The news came a day afterPresident Barack Obama laid out his plan to "dismantle and ultimately destroy" ISIS, the terror group that calls itself the Islamic State, including authorizing airstrikes.

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