A CIA spokesman said a new intelligence assessment estimated that Islamic State could muster between 20,000 and 31,500 fighters across Iraq and Syria, up from a previous figure of 10,000.
The new assessment is based on a review of intelligence reports from May to August. It is larger than the 20,000 figure being used by many outside experts.
CIA spokesman Ryan Trapani said the new total reflected stronger recruitment by the extremist group since June following battlefield successes and the group's declaration of a caliphate. He says the revised number also reflects "greater battlefield activity, and additional intelligence."