A CIA spokesman says a new intelligence assessment estimates that the Islamic State group can 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 says the new total reflects 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.”