PARIS—France called for coordinated international action to cut off the Islamic State's access to financial resources amid fears that the Sunni insurgency has become the world's richest jihadist movement.

French Foreign Minister Laurent Fabius said the extremist group, already a formidable military force, has enhanced its power by amassing huge piles of cash, thanks in part to individual donors from the Middle East.

"The attacks in New York on Sept. 11, 2001, cost $1 million," Mr. Fabius told French radio RMC on Friday. "Today, we estimate the Islamic State has several billions."

With the insurgency's rapid expansion this summer threatening Iraq's government, the U.S. is conducting airstrikes in northern areas of Iraq controlled by the Islamic State, while France and other European countries have decided to supply weapons to Iraq's Kurdish forces.

Mr. Fabius said that beyond military support, Western and Middle East countries must agree on measures to dry up the Islamic State's finances.