"The CIA assesses the Islamic State of Iraq and the Levant (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, an increase from our previous assessment of at least 10,000 fighters," a CIA spokesperson said.

"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," the spokesperson said.

The CIA estimate came a day after President Barack Obama on Thursday announced a major expansion of US-led military campaign, including American airstrikes in Syria and the deployment of 475 more military advisers to Iraq, to ‘degrade and ultimately destroy’ the Islamic State, also known as Islamic State of Iraq and Syria or ISIS or ISIL.

ISIS is an al-Qaeda splinter group and it has seized hundreds of square miles in Iraq and Syria, which is equal in size as that of Britain and poses a threat to the region.

Al-Qaeda has distanced itself from the group, chiding it for its lack of teamwork in its aggressive, brutal expansion.

US Secretary of State John Kerry on Thursday said that US is not at war with ISIS.

"What we are doing is engaging in a very significant counter-terrorism operation," Kerry said.

"It's going to go on for some period of time. If somebody wants to think about it as being a war with ISIL, they can do so, but the fact is it's a major counterterrorism operation that will have many different moving parts," Kerry said.

"ISIL is an animal unto itself. And it is significantly such a threat because of the foreign fighters that are attracted to it - which you don't see in Somalia or Yemen," he said, adding that ISIL has attracted a ‘significant coalition’ that is determined to go and destroy it.

US Defense Secretary Chuck Hagel spoke with his French counterpart Jean-Yves Le Drian to discuss the coalition strategy to combat the Islamic State, the Pentagon said.

Meanwhile, US Central Command continued to attack IS terrorists in Iraq, using attack aircraft to conduct two airstrikes on Wednesday and Thursday near the Mosul Dam.

US Central Command has conducted a total of 156 airstrikes across Iraq.

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