His statement was reinforced by newly obtained internal ISIS documents. The cache shows the group struggling for funds -- some of which are used to pay for sex slaves -- and calling on fighters to use less electricity and stop driving official cars for personal use. The fighters, meanwhile, seem to be suffering low morale, in some case seeking doctors' notes to avoid serving on the frontlines.
Maj. Gen. Peter E. Gersten told reporters that attacks on ISIS finances and personnel had reduced the number of foreign fighters joining ISIS from 1,500-2,000 per month a year ago to 200 per month today.
"We're actually seeing an increase in now the desertion rates in these fighters. We're seeing a fracture in their morale. We're seeing their inability to pay. We're watching them try to leave Daesh," Gersten said, using another name for ISIS.
Since October, the U.S.-led anti-ISIS coalition has been targeting the group's oil infrastructure and cash storage facilities in an effort to undermine ISIS' finances. Gersten said that the strikes had destroyed between $300 million to $800 million and promised additional strikes on ISIS finances.
"If it's one dollar bill on the street that they're using to build a weapon, I'm going to go after that one dollar," he said.
Aymenn al-Tamimi, the Jihad-Intel Research Fellow at the Washington-based Middle East Forum think tank, who obtained the ISIS documents, wrote in a paper accompanying their release Friday that, "The internal records make clear these pressures have been felt in the group's military, financial, and administrative domains."
Al-Tamimi acquired the documents from activists, journalists and former residents of the area controlled by the Islamic State. Some of the documents were recovered from areas recently liberated by local U.S. allies from ISIS control.
The memos were first published in the CTC Sentinel, a product of the Combating Terrorism Center at West Point overseen by CNN terrorism analyst Paul Cruickshank.
"While it was not feasible for Aymenn al-Tamimi to verify the documents with ISIS administrators in Syria, careful examination by al-Tamimi of their content, format and appearance found them to be consistent with documents drafted by ISIS in the past and generated no red flags," Cruickshank said.
The official ISIS records give an up-to-date look at the terrorist organization, with some of them dated as recently as March 2016.
Cash flow problems
The documents detail how ISIS fighters are being paid, with fighters receiving additional stipends for their wives, children and sex slaves.
The average fighter is receiving "$50 a month, with an additional $50 for each wife, $35 for each child, $50 for each sex slave, $35 for each child of a sex slave, $50 for each dependent parent, and $35 each for other dependents," according to the document.
Al-Tamimi said in the report that he received confirmation of these pay scales through conversations with ISIS-linked fighters during a visit to Syria.
The internal directives also call on ISIS fighters to reduce their electricity consumption and to stop driving official vehicles for personal use, a curbing of "perks" al-Tamimi said was indicative of an organization under financial strain.
Documents obtained earlier had shown that ISIS members stationed in Raqqa, Syria, ISIS' de facto capital, had to undergo a 50% pay cut.
Perhaps even more troubling for ISIS, the documents indicate that the organization is struggling to field adequate troops.
Al-Tamimi's paper said that the internal records show failed attempts to call-up additional fighters in the wake of defeats at al-Shaddadi in northeastern Syria and during the battle for Palmyra in central Syria.
Al-Tamimi also noted that in October, ISIS had "issued a general amnesty for deserters" in a bid to bolster its ranks.
The manpower shortage was fueled in part because ISIS fighters were trying to get fake notes from doctors in order to get out of combat, according to some records.
But these ISIS fighters might find such notes hard to come by, as al-Tamimi also said that the Islamic State was facing a "medical brain-drain" as doctors flee ISIS-held territory due to mistreatment by the terrorist group.
Despite these shortages, al-Tamimi and other experts do not envision a widespread revolt against ISIS. "Populations under Islamic State rule are accustomed to poor living standards, exacerbated by years of civil war, and will likely stomach further decreases in quality of life for the time being rather than rebel and risk a brutal crackdown," he wrote.
Gersten said, "The men and women of the coalition are fighting every day to destroy this cancer, but we also must understand that this fight requires both patience and time."