The Isis can muster up to three times the number of fighters than previously thought, estimates the US Central Intelligence Agency.
An analysis by the agency suggests the group may have anywhere between 20,000 and 31,000 jihadists.
Isis, which has captured vast swathes of Iraq and Syria, was initially thought to have had about 10,000 fighters.
A statement released by CIA spokesperson Ryan Trapani said: "CIA assesses the Islamic State of Iraq and the Levant 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."
"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 source of the group's strength is unclear and it is not known if it has made fresh recruitments in Iraq.
A CIA source told CNN that the Islamist fighters of the group come from more than 80 countries.
The CIA assessment comes in the wake of President Barack Obama's decision to step up the US military campaign in Iraq and Syria.
The US airstrikes will also begin targeting the top leadership of Isis.
"One of the ways you get at and you destroy the capabilities of an enemy like [Isis] is to be pretty aggressive against them, and that does include disrupting their ability to command and control and to lead their own forces," said Pentagon press secretary Rear Admiral John Kirby.