Islamic State militants in Iraq and Syria now have about 20,000 to 31,500 fighters on the ground, the Central Intelligence Agency said Thursday, much higher than a previous estimate of 10,000.

"CIA assesses the Islamic State of Iraq and the Levant (IS) can muster between 20,000 and 31,500 fighters across Iraq and Syria, based on a new review of all-source intelligence reports from May to August," CIA spokesman Ryan Trapani said in a statement.

"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," he said.

It comes a day after President Barack Obama vowed to expand an offensive against IS extremists, a plan which foresees new air strikes against IS in Syria, expanded attacks in Iraq and new support for Iraqi government forces. (***)