media captionJoe Biden: 'I stand squarely behind my decision'

President Joe Biden has said he stands "squarely" behind the US exit from Afghanistan as he faces withering criticism over the Taliban's lightning conquest of the war-torn country.

"How many more American lives is it worth?" said the president.

He said despite the "gut-wrenching" scenes in Kabul "there is never a good time to withdraw US forces".

On Sunday the Taliban declared victory after Afghan President Ashraf Ghani fled and his government collapsed.

The militants' return to rule brings an end to almost 20 years of a US-led coalition's presence in the country.

Kabul was the last major city in Afghanistan to fall to a Taliban offensive that began months ago but accelerated in recent days as they gained control of territories, shocking many observers.

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Mr Biden returned on Monday to the White House from the Camp David presidential retreat to make his first public remarks on Afghanistan in nearly a week.

"If anything, the developments of the past week reinforce that ending US military involvement in Afghanistan now was the right decision," said Mr Biden.

"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."

Mr Biden is facing intense political backlash over his April decision to order all American troops out of Afghanistan by 11 September – the 20 year anniversary of the terror attacks that triggered the US invasion.

On Monday he said the US mission in Afghanistan was never supposed to have been about nation-building.

media captionPeople tried to cling on to the wheels of a departing US military plane

He said he had opposed the 2009 deployment of thousands more troops into the country by former President Barack Obama when he was vice-president.

Mr Biden also noted he had inherited a deal negotiated with the Taliban under former President Donald Trump for the US to withdraw from Afghanistan by May of this year.

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He said he was now the fourth US president to preside over America's longest war.

"I will not pass this responsibility on to a fifth president," said Mr Biden, a Democrat.

"I will not mislead the American people by claiming that just a little more time in Afghanistan will make all the difference."

Mr Biden campaigned as a seasoned expert in foreign policy and declared after assuming office this year that "America is back".

Last month he assured reporters it was "highly unlikely" the Taliban would overrun the entire country.

Earlier on Monday, US National Security Adviser Jake Sullivan told NBC that Afghanistan fell because the people lacked the "will" to defend themselves from the Taliban.