The CIA believes that there may be as many as 31,000 Islamic State fighters spread across Iraq and Syria – three times as many as previously estimated.
The new assessment comes just days after US president Barack Obama authorised an open-ended campaign of air strikes against IS.
The CIA had previously thought that the militant group had around 10,000 fighters, but new intelligence reports dating from May to August indicate that numbers may have risen due to successful recruitment.
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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," CIA spokesman Ryan Trapani said.
America's Arab allies, Saudi Arabia, Iraq, Jordan, Lebanon, Egypt and six Gulf states have offered swift "if vague" support for Obama's new strategy, The Guardian reports.
John Kerry, the US Secretary of State, held talks with representatives of the Arab states who agreed to support the military campaign "as appropriate" and help rebuild communities "brutalised" by IS.
The new Iraqi government expressed its support for the campaign: "We welcome this new strategy," said Hoshyar Zebari, one of Iraq's newly appointed deputy prime ministers. "There is an urgent need for action. People cannot sit on the fence. This is a mortal threat to everybody."
However, Russia, Iran and Syria warned that without prior approval from the UN, an offensive against IS within Syria would violate international law.
Russian foreign ministry spokesman Alexander Lukashevich said: "The US president has spoken directly about the possibility of strikes by the US armed forces against Islamic State positions in Syria without the consent of the legitimate government. This step, in the absence of a UN Security Council decision, would be an act of aggression, a gross violation of international law."
The US military is already active in Iraq, where it has mounted 150 air strikes against IS militants. The BBC says that the US has already sent hundreds of military advisors to assist the Iraqi government and Kurdish fighters. Other countries, including the UK, are providing humanitarian assistance to Iraqi people displaced by the conflict.
