The CIA said Thursday that the number of Islamic State fighters has grown to as many as three times previous estimates, a day after President Obama announced he is drastically expanding the U.S. military campaign against the militants.

A spokesperson for the agency told Fox News that new assessments show that the militant group can muster between 20,000 and 31,500 fighters across Iraq and Syria.

The spokesperson said the new figures were determined after a review of “all-source intelligence reports” on the group from May to August.

The new numbers are a big jump from the agency’s previous estimate that there were at least 10,000 Islamic State fighters.

The spokesperson said the increase is likely due to the militant’s group recruiting gains after its success on the battlefield.

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“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.

Obama announced Wednesday that he is launching an expanded campaign to destroy the Islamic State, and described a four-point “counterterrorism” strategy to fight the militants.

He authorized airstrikes in Syria, and expanded airstrikes in Iraq, and his administration already is drafting a target list of key leaders of the Islamic State organization.

Fox News' Catherine Herridge contributed to this report.