WASHINGTON, United States -- President Joe Biden defended the US pullout from Afghanistan Monday, saying he stood by the policy and that it was time to leave after 20 years of conflict.

"I am president of the United States of America and the buck stops with me," Biden said in a much-awaited televised address from the White House, after several days of silence on the momentous developments.

As scenes of mayhem unfolded in the Afghan capital, Biden said he was "deeply saddened" by the turn of events -- and promised to "speak out" on the rights of women now facing a return to Taliban rule.

But he was steadfast in insisting he did not regret pulling out America's troops -- despite a torrent of criticism of the chaotic end to two decades of US-led military intervention.

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

The US leader acknowledged that the Afghan government collapsed more quickly than he expected -- and suggested that they had lacked the will to stand up to the Taliban.

"The truth is, this did unfold more quickly than we had anticipated," Biden said.

"We gave them every chance to determine their own future. We could not provide them with the will to fight for that future."

Biden reiterated however that the US national interest in Afghanistan was always principally about preventing terrorist attacks on the US homeland -- and that America would continue to "act quickly and decisively" against any terror threat emanating from the country.

"The mission in Afghanistan was never supposed to be nation-building," he said.

And the US president issued a stark warning to the Taliban not to disrupt or threaten the evacuation of thousands of American diplomats and Afghan translators at the Kabul airport.

"We will defend our people with devastating force if necessary," he said.