U.S. Army Gen. Scott Miller, the top U.S. commander in Afghanistan, speaks at a ceremony where he relinquished his command, at…
U.S. Army Gen. Scott Miller, the top U.S. commander in Afghanistan, speaks at a ceremony where he relinquished his command, at Resolute Support headquarters, in Kabul, Afghanistan, July 12, 2021.

PENTAGON - The top U.S. general in Afghanistan stepped down from his post Monday, as Taliban fighters continue to make gains across the war-torn country.  

The Biden administration has said the official end date of the U.S. troop withdrawal will be August 31, but General Austin "Scott" Miller's relinquishing of command was a symbolic end to the troop pullout.  

"The people of Afghanistan will be in my heart and on my mind for the rest of my life," Miller said during a short ceremony at the top U.S. military headquarters in Kabul. 

U.S. Central Command (CENTCOM) chief, General Frank McKenzie, arrived in the Afghan capital early Monday and took command of the remaining forces. 

Marine Gen. Frank McKenzie, the head of U.S. Central Command, attends a ceremony where Gen. Scott Miller, not pictured, handed over command, at Resolute Support headquarters, in Kabul, Afghanistan, July 12, 2021.

The withdrawal is "more than 90%" complete, according to U.S. Central Command. Most of the U.S. troops and equipment have left, with fewer than 1,000 troops remaining to protect the U.S. Embassy and help with securing the international airport in Kabul.  

McKenzie already had authority over U.S. military operations in Afghanistan and several other neighboring nations as head of CENTCOM, and he will continue his oversight from his headquarters in the United States while two-star officer, Rear Admiral Peter Vasely, helps oversee the mission on the ground.  

Miller was the longest-serving senior U.S. military officer of the Afghan war. He served for about three of the nearly 20 years of U.S. military involvement, overseeing the drawdown after the Trump administration's February 2020 deal with the Taliban and the final withdrawal called for by President Joe Biden in April.  

"As I depart Afghanistan, I believe that it's very appropriate to remember sacrifice," Miller said at the transfer of command ceremony. 

"Our job is now just not to forget," he said, acknowledging all Americans, Afghans, and international service members and civilians who lost their lives in Afghanistan. 

WATCH: US Afghan commander steps down

U.S. troops first entered the country in 2001 to overrun bases where al-Qaida terrorists trained to launch the September 11, 2001, hijacked airliner attacks against the United States that killed nearly 3,000 people. 

McKenzie will still be able to support the Afghan government with U.S. airstrikes through the end of August against the Taliban, who were toppled in 2001 for harboring al-Qaida.  

But after the withdrawal's completion, U.S. strikes in Afghanistan will solely support counterterrorism operations against al-Qaida and Islamic State, he told VOA in a recent interview. The U.S. military will continue to help the Afghan military financially and by aircraft maintenance support from outside Afghanistan.  

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Taliban insurgents say they already control 85% of the country, a claim that has been contested. But since the official start of the withdrawal on May 1, the Taliban have nearly tripled the number of districts it controls, from about 75 to now more than 210 of Afghanistan's 407 districts, according to the Foundation for Defense of Democracies' Long War Journal.    

"What I tell the Taliban is, they're responsible, too. The violence that's going on is against the will of the Afghan people, and it needs to stop," Miller said Monday. 

On Sunday, Pentagon press secretary John Kirby told "Fox News Sunday" the U.S. is "watching with deep concern" as Taliban insurgents take control of more and more territory in Afghanistan, while U.S. forces are quickly returning home under Biden's withdrawal orders.  

Biden last week staunchly defended the U.S. troop withdrawal, even in the face of Taliban advances.   

"We did not go to Afghanistan to nation-build," Biden said at the White House. "It's the right and the responsibility of the Afghan people alone to decide their future and how they want to run their country."   

He said the U.S. went to Afghanistan to bring former al-Qaida leader Osama bin Laden through "the gates of hell" and to eliminate al-Qaida's capacity to deal more attacks against the United States.  

"We accomplished both of those," Biden said. "That's why I believe that this is the right decision, and quite frankly, overdue."  

Ken Bredemeier contributed to this story