US intelligence service, 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.
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.