WASHINGTON: 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.

Among those in Syria are 15,000 foreign fighters including 2,000 Westerners, some of whom have joined IS, a US official said.

The figures were revealed one 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.

“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.

Also on Thursday the Pentagon announced that US combat aircraft will soon start flying out of a base in the Kurdish region of northern Iraq as part of a “more aggressive” air campaign against IS jihadists.

The use of Arbil air base reflects the broadening US offensive, though attack helicopters already have been flying out of bases in Iraq.