A series of U.S. airstrikes in recent months have destroyed "hundreds of millions" of dollars in Islamic State cash as part of a broader campaign aimed at squeezing the extremist group's financial power, a U.S. military spokesman said Wednesday.

Col. Steve Warren, speaking for the coalition that is fighting the Islamic State in Iraq and Syria, said the effort -- which has included airstrikes against oil processing and distribution facilities mainly in Syria -- has forced the Islamic State to adjust by reducing salaries for its fighters.

The Associated Press reported on Tuesday that the Islamic State is having a hard time meeting expenses. It reported that the group has slashed salaries across the region, asked residents of Raqqa, its de facto capital in Syria, to pay utility bills in black market American dollars, and is now releasing detainees for a price of $500 a person.

Warren said that in some cases IS has reduced salaries to fighters by as much as 50 percent.

"This, to us, is one indicator that these strikes against their ability to generate revenue are beginning to squeeze them a little bit," he said.

Warren had previously estimated that strikes against IS cash holdings had eliminated tens of millions of dollars. On Wednesday, he said the latest estimate is hundreds of millions, but he would not offer a more specific total.

Of a total of 10 U.S. airstrikes against IS cash holdings since October, seven have been in Iraq and three in Syria. The first was Oct. 21, followed by other single strikes on Nov. 14, Dec. 16, Jan. 11 and Jan. 18. The campaign was intensified last weekend with a series of five airstrikes near Mosul, in northern Iraq, hitting what Warren described as two IS financial distribution centers and two financial storage centers.

Separately, the Pentagon said Wednesday that the air campaign against IS has cost a total of $6.2 billion since it began in August 2014. The daily cost is now averaging $11.5 million.