An Islamic State flag flies over the custom office of Syria's Jarablus border gate as it is pictured from the Turkish town of Karkamis, in Gaziantep province, Turkey in this August 1, 2015 file photo. REUTERS/Murad Sezer/Files

WASHINGTON (Reuters) - U.S.-led air strikes on Islamic State cash storage sites have cost the militant group hundreds of millions of dollars, a U.S. military spokesman said on Wednesday.

The United States is trying to cut revenue to Islamic State - believed to be one of the best-funded militant groups in the world - through air strikes targeting its oil production as well as cash storage sites. U.S. officials believe the ultra-hardline Sunni group is more dependent on cash as it has seen its access to the formal banking system reduced through sanctions and other measures.

Colonel Steve Warren, a spokesman for the U.S.-led military effort against Islamic State, said the air strikes on cash storage and collection sites have destroyed hundreds of millions of dollars’ worth of currency. He declined to give a more precise estimate of how much money had been destroyed.

As of mid-January, the U.S.-led coalition had struck 10 Islamic State cash storage and distribution sites in Iraq and Syria, Warren said last month.

In addition to selling oil pumped from territory it controls, Islamic State also has earned revenue through taxation, selling antiquities and raids on banks in its territory, U.S. officials say.

Islamic State has declared a self-styled caliphate across areas of territory it controls in Iraq and Syria, imposing its own harsh interpretation of Islamic law.

U.S.-led air strikes have helped force the militant group to cut its fighters’ pay by up to 50 percent, a senior U.S Treasury official said earlier this month.