CENTCOM Commander Gen. Kenneth McKenzie said the Taliban were "helpful and useful when we closed down operations."

"They established a firm perimeter outside of the airfield to prevent people from coming on the airfield during our departure," McKenzie said of the Taliban Monday. "They did not have direct knowledge of our time of departure, we chose to keep that information very restricted. But they were actually very helpful and useful to us as we closed down operations."

McKenzie's comments come as the news broke Monday that the last U.S. forces in Afghanistan had departed the country, officially bringing the 20-year conflict to an end.

LAST US TROOPS HAVE DEPARTED AFGHANISTAN

The removal of U.S. forces met the Aug. 31 timeline that the Biden administration had agreed to with the Taliban, though the White House did acknowledge Monday that a "small number" of Americans were not able to make it out of the country. 

"We believe there are still a small number" left in the country, White House Press Secretary Jen Psaki said, though she could not say how many. A senior State Department official put the number at "below 250."

McKenzie said that no civilian U.S. citizens were aboard the final five military flights leaving Kabul.

"There’s a lot of heartbreak associated with this departure," McKenzie said. "We did not get everybody out that we wanted to get out."

The U.S. also left behind equipment such as the C-RAM (counter-artillery, artillery and mortar) system that was used to shoot down rockets, armored Humvees, and some aircraft, though McKenzie said the equipment was no longer mission capable.

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The general said that the ISIS threat to the evacuation remained "very real" until the end, with "overwhelming" U.S. airpower hovering above Kabul at the ready to address the threats.