The CIA says the Islamic State (IS) militant group may have up to 31,000 fighters in Iraq and Syria - three times as many as previously feared.
A spokesman said the new estimate was based on a review of intelligence reports from May to August.
IS has seized vast swathes of Iraq and beheaded several hostages in recent months, leading to US airstrikes.
US Secretary of State John Kerry is visiting Turkey, seeking more support for action against IS.
US officials say retired Gen John Allen will be tasked with forming an international coalition to fight IS.
On Thursday, 10 Arab countries agreed to help the US attack the group in both Iraq and Syria.
The CIA had previously believed that IS had about 10,000 fighters, spokesman Ryan Trapani 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," he added.
The revision comes a day after President Obama outlined a plan to "degrade and destroy" IS and to increase military support for allied forces engaged in fighting the group. For the first time, he authorised air strikes against the group in Syria.
In recent months IS has expanded from its stronghold in eastern Syria and seized control of more towns, cities, army bases and weaponry in Iraq.
The US has already carried out more than 150 air strikes against IS in Iraq. It has also sent hundreds of military advisers to assist Iraqi government and Kurdish forces, but has ruled out sending ground troops. Other countries, including the UK, have contributed humanitarian assistance to Iraqis displaced by the group's advance. Mr John Kerry secured the co-operation of several Arab countries during a meeting in Jeddah, Saudi Arabia on Thursday.
Nato member Turkey, however, refused to sign a communique calling for countries to join the US in the fight against IS. Analysts say this may be because the group currently holds 49 Turkish citizens, including diplomats. Mr Kerry downplayed the move, saying the important US ally was dealing with some "sensitive issues".
He is due to travel to Turkey on Friday to try to secure more co-operation from the government.