The CIA estimates the Islamic State in Iraq and Syria (ISIS) is made up of anywhere between 20,000-31,500 fighters, according to reports Thursday night.
The agency previously put the number at 10,000 but revised it upward after stronger recruitment since June, according to CIA spokesman Ryan Trapani, who was quoted in The Associated Press.
Among them are hundreds of foreign fighters, including around a dozen known U.S. fighters.
The militant group's success on the battlefield and its leaders' assertion that it has established a caliphate led to the surge in recruitment, the CIA official said.
The group has taken over broad swaths of territory in Iraq and Syria this year and received universal condemnation in Washington after beheading two U.S. journalists.
The CIA number was unveiled a day after President Obama outlined a plan to destroy the group by broadening its air campaign in Iraq and extending it into Syria. The United States also plans to increase its military support to Iraq fighters and vetted Syrian rebel groups, along with other allies.