U.S. President Joe Biden on Monday defended the withdrawal of American troops from Afghanistan by saying U.S. troops could not defend a nation whose leaders "gave up and fled."

"We gave them every chance to determine their own future. We could not provide them with the will to fight for that future," Biden said, adding he could no longer ask U.S. soldiers to risk their lives in the country, 20 years on. "Our mission in Afghanistan was never supposed to have been nation-building." 

In his address at the White House – his first public appearance since Taliban took control of the country during the weekend – he admitted the Taliban advance had unfolded more quickly than expected. 

In just over a week, the Taliban, which the U.S. overthrew in 2001, went from winning control of their first provincial capital to taking over all the major cities including capital Kabul. 

Saying he was willing to take criticism over the decision on troops withdrawal, Biden also blamed top Afghan leaders, Afghan forces and his predecessor Donald Trump for the unfolding crisis in Afghanistan. 

"We have to be honest, our mission in Afghanistan is taking many missteps – made many missteps over the past two decades... I know my decision will be criticized, but I would rather take all that criticism than pass this decision on to another president... yet another one, a fifth one," he said.

Trump, who inked a deal with the Taliban to withdraw U.S. forces by this past May, also bears part of the responsibility for the current situation, said Biden. 

"The choice I had to make as your president was either to follow through on that agreement or be prepared to go back to fighting the Taliban in the middle of the spring fighting season," Biden said. "There would be no ceasefire after May 1." 

Biden said the U.S. mission in Afghanistan to defeat the September 11 attackers and ensure al-Qaida could not use Afghanistan as a base to hit the United States had been completed. 

"I made a commitment to the American people when I ran for president that I would bring America's military involvement in Afghanistan to an end. While it's been hard and messy and, yes, far from perfect," he said. 

Biden said the United States will evacuate from Afghanistan thousands of American citizens who have been living and working in the country, warning the Taliban of "swift and forceful" response if these efforts come under attack. 

The United States would expand refugee access to include other vulnerable Afghans not already covered by special immigrant visas, volunteers for nongovernmental organizations and personnel from U.S. news agencies, he said.

(With input from agencies)