Pentagon spokesman John Kirby said thousands of ISIS-K prisoners were released in Afghanistan.
He blamed Afghan forces for their lack of resistance as the Taliban took Bagram Air Base.
ISIS-K claimed responsibility for a bombing at the Kabul airport on Thursday that left scores dead.
Pentagon spokesman John Kirby said at a press conference on Friday that thousands of ISIS-K prisoners were released in Afghanistan after the government fell to the Taliban two weeks ago.
"I don't know the exact number. Clearly, it's in the thousands when you consider both prisons, because both of them were taken over by the Taliban and emptied. But I couldn't give you a precise figure," he said in response to a reporter who asked how many how many prisoners were left at Bagram Air Base.
Kirby said the US military was turning over its entities to Afghan security forces and blamed them for a lack of resistance as the Taliban advanced.
"And as for emptying out, remember we were turning things over to Afghan national security forces, that was part of the retrograde process, was to turn over these responsibilities. And so they did have responsibility for those prisons and the bases at which those prisons were located," Kirby said. "And of course as the Taliban advanced, we didn't see the level of resistance by the Afghans to hold some territory, some bases, and unfortunately those were the bases the Afghans didn't hold."
ISIS-K likely carried out Thursday's bombing outside the Kabul airport
ISIS-K, the Islamic State's Afghanistan affiliate, claimed responsibility for an attack in Kabul on Thursday that killed at least 13 US service members and wounded 18 more. More than 170 people were killed and at least 200 were wounded in the blast, an official with Afghanistan's Ministry of Public Health told CNN.
The Taliban and ISIS-K are sworn enemies. They've been fighting for years. ISIS-K, which arose in 2015, views the Taliban as apostates and not devout enough it terms of its adherence to Islam.
Though ISIS-K fighters were released during the Taliban's final, rapid push for control of the country, the Taliban also made a point to execute ISIS-K's former leader in the process. After the Taliban marched into the capital, Abu Omar Khorasani - the former head of the Islamic State's Afghanistan affiliate - was taken from Pul-e-Charkhi by Taliban militants and promptly killed alongside eight other ISIS-K members, the Wall Street Journal reported.
Experts say ISIS-K has an interest in generating chaos in Afghanistan to embarrass the Taliban and undermine its legitimacy.
The Taliban's takeover of Afghanistan "puts significant pressure on ISIS to demonstrate its continued relevance to global jihad, which will make ISIS more dangerous as it attempts to prove the organization's capability and relevance," Jennifer Cafarella, a national security fellow at the Institute for the Study of War, told Insider.
"ISIS will attempt to erode the Taliban's governance and attack the Taliban's religious legitimacy on the ground," Cafarella added.
The group is assessed to have somewhere between 1,500 to 2,000 fighters in Afghanistan, according to a UN report from June, though its numbers may have been bolstered by the recent prison breaks.
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