A day after U.S. President Obama's announcement of an expanded counter-terrorism campaign, the Central Intelligence Agency (CIA) released information on Thursday that the Islamic State (ISIL) could raise between 20,000 and 31,500 extremist fighters, a figure three times more than the initial estimates.

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Previous estimates have pinned ISIL at 10,000 strong, including Sunni loyalists who have joined the extremist group as it advanced across Iraq.

But after ISIL's declaration of a Muslim caliphate in June, acquisition of additional intelligence capabilities, and a string of victories on the ground, it has recruited at least 15,000 foreign fighters into its ranks, a CIA source told CNN.

At least 2,000 of these recruits came from Western countries, the rest were from about 80 other countries, the unnamed source added.

The new estimates are based on all intelligence sources that were gathered from May to August, reported the Press Trust of India.

It remains unclear how the number of fighters have swelled. Neither is it clear whether the new recruits came primarily from within Iraq or were active ISIL fighters, CNN noted.

The new CIA information followed news that the United States have authorized surveillance flights across Syria, which according to CNN, is part of the intelligence gathering to determine possible airstrike targets.

As Pentagon works to identify targets, the U.S. air campaign sends about 60 reconnaissance flights per day over Iraq, CNN reported.

On Thursday, the Pentagon announced the United States' plan to send armed and manned warplanes in ISIL-controlled territories.

Also on Thursday, Saudi Arabia with 9 other Gulf and Arab states have signed a joint agreement to support the anti-ISIL offensive U.S. and its growing international allies have launched against the terrorist group.

Included in the Arab-backed broad counterterrorism strategy is the use of Saudi Arabia's military bases to train moderate Syrian insurgents. The Gulf leaders have likewise agreed to provide reconnaissance support to the international anti-ISIL coalition.

The rest of the anti-ISIL coalition members are expected to finance the military campaigns the Kurds, Iraqi troops and Syrian rebels are conducting on the ground.

TagsCIA, ISIL, ISIS, anti-ISIS offensive, anti-ISIL coalition, anti-ISIL campaign