DHAKA: Islamic State militants in Iraq and Syria now have about 20,000 to 31,500 fighters on the ground.

The Central Intelligence Agency said this which is 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 intelligence 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, reports the Times of India.

“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.

The White House has insisted that President Barack Obama is authorized to strike IS in Iraq and Syria under a law passed by Congress after the September 11 attacks in 2001.

The announcement came on the 13th anniversary of the September 11 attacks in 2001 — a day that fell, in a trace of bitter irony, only hours after Obama's remarks steeling Americans for a battle against Muslim fanaticism in the Middle East.

BDST: 1709 HRS. SEP 12, 2014