Biden said the Taliban takeover unfolded "more quickly than we had anticipated" and placed blame on the Afghan government and military for an apparent lack of resistance.

"So what's happened? Afghanistan's political leaders gave up and fled the country. The Afghan military collapsed, sometimes without trying to fight," he said.

"If anything, the developments of the past week reinforced that ending U.S. military involvement in Afghanistan now was the right decision. American troops cannot and should not be fighting in a war and dying in a war that Afghan forces are not willing to fight for themselves," the president added.

"We gave them every tool they could need. We paid their salaries," Biden said. "We gave them every chance to determine their own future. We could not provide them the will to fight for that future."

Biden countered those who criticize his decision with questions, asking, "How many more generations of America's daughters and sons would you have me send to fight Afghanistan's civil war when Afghan troops will not? How many more lives, American lives, is it worth? How many endless rows of headstones at Arlington National Cemetery?"

"I'm clear on my answer," Biden said. "I will not repeat the mistakes we've made in the past."

Speaking for the first time publicly since the Taliban seized control of Kabul, Biden defended his decision to withdraw troops from Afghanistan, while acknowledging the Taliban takeover "occurred more quickly" than the administration had foreseen.

"Our mission in Afghanistan was never supposed to have been nation-building. It was never supposed to be creating a unified centralized democracy. Our only vital national interest in Afghanistan remains today what it has always been, preventing a terrorist attack on American homeland," Biden said.

"As president, I am adamant we focus on the threats we face today in 2021, not yesterday's threats," he added.

Biden repeated that America's longest war -- and the plan to withdraw troops from it -- was one he inherited from the Trump administration.

"I stand squarely behind my decision," he continued. "After 20 years, I've learned the hard way that there was never a good time to withdraw U.S. forces."

The remarks come amid chaotic evacuations of U.S. personnel and Afghan allies from the capital city.

Biden returned from Camp David to Washington Monday afternoon to address the nation from the White House East Room on the worsening crisis in Afghanistan -- the first time the president has spoken publicly on the situation since last Tuesday.

"They've got to fight for themselves -- fight for their nation," Biden said of the Afghan people Tuesday.

He has insisted that despite rapid advancements by the Taliban, he does not regret the decision to withdraw U.S. troops from the country and that it is time for Afghan forces to defend their own homeland. He has also attempted to shift responsibility for the fall of Afghanistan to his predecessor, claiming the Trump administration empowered the Taliban.

"One more year, or five more years, of US military presence would not have made a difference if the Afghan military cannot or will not hold its own country," Biden said in a statement over the weekend. "And an endless American presence in the middle of another country's civil conflict was not acceptable to me."

U.S. forces in Kabul are "working to reestablish security at Hamid Karzai International Airport following breaches overnight," Pentagon spokesman John Kirby said earlier Monday.

"We do not know how long this will take certainly seeing all the dramatic video coming from the airport today and we obviously don't want anyone else to get hurt," he told reporters in an off-camera briefing.

Kirby confirmed there were security incidents at the airport Sunday night "involving armed individuals, shooting at U.S. forces." He said the U.S. responded in self-defense with fatal fire.

"I want to reiterate that while our mission is not offensive, forces have the inherent right of self-defense, and they will respond accordingly to threats and attacks to separate incidents," he said. "U.S. forces did respond to hostile threats that resulted in the death of two armed individuals on the support for vulnerable or at-risk Afghan citizens."