ISIL in Iraq and Syria now has 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.

ISIL in Iraq and Syria now has 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.

Among those in Syria are 15,000 foreign fighters including 2,000 Westerners, some of whom have joined ISIL, a US intelligence official told AFP.

The figures were revealed one day after President Barack Obama vowed to expand an offensive against ISIL extremists, a plan which foresees new air strikes against ISIL in Syria, expanded attacks in Iraq and new support for Iraqi government forces.

"CIA assesses ISIL 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.

Senior US officials have voiced concern at the presence of foreign fighters among the ISIL terrorists who hold Western passports, potentially enabling them to return from the battlefield prepared to carry out terror attacks in Europe or the United States.