Chairman of Joint Chiefs of Staff Gen. Mark Milley warned lawmakers during a briefing over the weekend that terror groups like al-Qaeda may be able to grow at a much faster pace and pose a threat to the United States in the wake of the Taliban takeover of Afghanistan, a source confirmed to Fox News.

The Associated Press first reported Monday that Milley, during a briefing with senators on Sunday, said U.S. officials were expected to alter their earlier assessments about the pace of terrorist groups rebuilding in Afghanistan.

Earlier this summer, the Pentagon’s top leaders warned that extremist groups, like al-Qaida and ISIS, could have the potential to regenerate in the region and pose a threat to the U.S. homeland within two years of the withdrawal of U.S. troops from Afghanistan.

But amid the worsening security situation in Afghanistan and the Taliban takeover, U.S. officials now believe terror groups may be able to grow at a much faster pace.

Last month, Milley warned that the Taliban had "strategic momentum" in the fight for control of Afghanistan, while maintaining that the withdrawal of U.S. troops would be completed by Aug. 31.

Defense Secretary Lloyd Austin, at the time, said that the focus of continued U.S. military efforts in Afghanistan would be countering terrorist threats, not the Taliban. Austin said the U.S. will "keep an eye on" Al Qaeda, the extremist network whose use of Afghanistan as a haven for planning the 9/11 attacks on the United States was the reason U.S. forces invaded Afghanistan in 2001.

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"Our major focus going forward is to make sure that violence, terrorism, cannot be exported from Afghanistan to our homeland, and so we’ll maintain the capability to be able to not only observe that but also address that if it does emerge," Austin said in July, adding that the Taliban pledged in 2020 to not provide a sanctuary for Al Qaeda in the future.

"We expect for them to meet that commitment. If they want legitimacy going forward, I think that’s something they’ll have to consider. That’s one way to earn it, so we’ll see what happens." At the time, he reiterated his view that there is a "medium risk" of Al Qaeda regaining within about two years of the U.S. departure the capability to launch attacks against the West.

"But, again, there are a number of things that could happen to speed that up a bit or slow it down," Austin said in July.

But the Taliban took over the entire country in mere days, killing American allies along the way. The U.S. is now struggling to keep the Kabul airport open for those still hoping to flee.

Meanwhile, President Biden had repeatedly dismissed the notion that without American support the country would fall into the hands of the brutal Taliban, which has a history of allowing terrorist organizations to flourish under its rule.

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As the Taliban took control of Kabul, the capital of Afghanistan, on Sunday, the terrorist group also seized Bagram Air Base, freeing thousands of U.S. prisoners, including some of the Taliban's most hardened fighters and senior Al Qaeda operatives.

U.S. forces handed control of the airfield, located on the outskirts of Kabul, over to the Afghan government in early July. Afghan forces at the base surrendered to the Taliban on Sunday, as other Taliban troops stormed the U.S. Embassy in Kabul and the presidential palace.

The Associated Press contributed to this report.