The U.S. Central Intelligence Agency says it now believes the number of Islamic State fighters in Syria and Iraq is somewhere between 20,000 and 31,500.
CIA spokesman Ryan Trapani said on September 11,"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."
Previously the number of IS fighters was estimated to be around 10,000.
The new figure is based on a review of intelligence reports from May to August this year.
U.S. President Barack Obama made an address to the nation on September 10 in which he vowed to eradicate the IS.
IS militants are responsible for killing thousands of people in Syria and Iraq, particularly Shi'ite Muslims and non-Muslims.