The Islamic State (IS) may have as many as 31,500 fighters in Syria and Iraq – three times higher than previously feared, the CIA believes.
A review of intelligence reports from May to August estimates the militant group can muster between 20,000 and 31,500 jihadists, up from a previous figure of 10,000, it said.
The new total reflects stronger recruitment by IS after its military successes and declaration of a caliphate in Iraq and Syria, said CIA spokesman Ryan Trapani.
US Secretary of State John Kerry is pressing Arab states to support an expanded American military campaign against IS, including airstrikes in Syria.
On Thursday, key Arab allies promised to “do their share” to fight IS, including stopping the flow of fighters and funding to the militants, but Nato member Turkey refused to join in.
The 10 Middle East allies announced their backing for a strategy to “destroy” the group “wherever it is, including in both Iraq and Syria,” following a meeting with Mr Kerry in the Red Sea city of Jiddah.
Nearly 40 countries have agreed to contribute to what Mr Kerry said would be a worldwide fight to defeat the militants.
“The devastating consequences of extremist hate remain fresh in the minds of all Americans, and to so many of our friends and allies around the world,” said Mr Kerry on the 13th anniversary of the September 11 terror attacks.
“Those consequences are felt every day here in the Middle East.”
Turkey attended the meeting but did not sign the final agreement over fears for 49 Turkish citizens kidnapped from its consulate in the northern Iraqi city of Mosul when it was overrun by IS in June.
US officials played down Turkey’s absence, noting it is still extremely concerned about the fate of its diplomats.
Some Gulf states could in theory help with US airstrikes, as the United Arab Emirates and Qatar did in the aerial campaign over Libya in 2011, as well as assisting with weapons, training, intelligence and logistics.
America has launched more than 150 airstrikes in Iraq over the past month, and has sent military advisers into the country.
On Monday, officials from the US, UK, France, Russia and China, and possibly other countries including Iran, are due to hold talks in Paris on how to stabilise Iraq .
