US President Joe Biden has threatened the Taliban with "a swift and strong" military response in the event of an attack on US forces.

This applies to any action by the Taliban in Afghanistan that would endanger US personnel or their mission, Biden said on Monday afternoon (local time) in the White House.

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

At the same time, the US President vehemently defended his decision to withdraw American troops from the crisis country. He's adamant about his decision, said Biden. It would have made no difference if the US troops had stayed a little longer in Afghanistan, he argued, and again accused the Afghan security forces of a lack of combat readiness. Biden admitted, however, that the United States had underestimated the speed of the Taliban's advance: "This has developed faster than we expected."

In the past few weeks, after the withdrawal of foreign troops, the Taliban had taken practically all provincial capitals of the country at breakneck speed - many without a fight.

They also entered Kabul on Sunday.

There were no fights.

The lightning advance surprised many observers, experts and also the US government.

On paper, the Taliban were inferior to the Afghan armed forces.

It is estimated that around 300,000 police and army personnel faced around 60,000 less well-equipped Taliban fighters.

However, these benefit from their brutal reputation, which they achieved during their rule in the 1990s with public executions or flogging.

Biden said the Afghan military had collapsed, "in part without attempting to fight." The US trained and equipped the Afghan security forces. But the United States could not have given them the will to fight for their future. Last week, Biden complained about the Afghan security forces' lack of willingness to fight.

Biden's address on Monday was his first public statement since the militant Islamist Taliban actually took power in Afghanistan and the capital Kabul. The US President emphasized that the original aim of the US operation in Afghanistan, the eradication of the terrorist group Al-Qaeda after the attacks of September 11, 2001, had long been achieved. The USA could effectively fight Islamist terrorist groups like Al-Qaida even without a permanent military presence in the target country. The US military shows this in other countries such as Somalia or Yemen. If necessary, this could also be done in Afghanistan in the future.

The Taliban had once granted refuge to al-Qaeda fighters and the then head of the terrorist organization, Osama bin Laden. The attacks by the terrorist group in the USA on September 11, 2001 triggered the US-led military operation in Afghanistan, which overthrew the Taliban who had ruled the country up until then. Bin Laden himself was killed in May 2011 during an operation by US special forces in Pakistan.