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?" asked the Democratic president.

He said that despite the "messy" pullout, "there was 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.

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 the turmoil in Kabul following 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.

Mitch McConnell, Republican Senate Minority Leader, tweeted: "What we are seeing in Afghanistan is an unmitigated disaster. The Biden Administration's retreat will leave a stain on the reputation of the United States."

Former US President George W Bush, who authorised the military intervention in 2001, said he was "watching the tragic events unfolding in Afghanistan with deep sadness".

"The Afghans now at greatest risk are the same ones who have been on the forefront of progress inside their nation," Mr Bush said, stressing that the US had "the legal authority to cut the red tape for refugees during urgent humanitarian crises".

In his speech, Mr Biden said the US mission in Afghanistan was never supposed to have been about nation-building.

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

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.

He said he was now the fourth US president to preside over America's longest war, and would not pass the responsibility on to a fifth.

"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.

But he conceded on Monday that "this did unfold more quickly than we had anticipated".

Opinion polls show most Americans support the US exiting Afghanistan.

But Mr Biden is facing a barrage of criticism over the manner of the departure after he withdrew US troops then sent thousands back in to help the evacuation.

Images emerged on Sunday showing US helicopters circling the US embassy in Kabul.

For many, the pictures evoked America's humiliating departure from Saigon, Vietnam, in 1975 when Mr Biden was a junior senator.

Earlier on Monday, the US suspended its evacuation from Kabul after scenes of panic at the capital's airport turned deadly.

Thousands of civilians desperate to flee the country had thronged the tarmac.

A US military official told the BBC's US partner CBS News that American troops had killed two armed Afghans who were part of a huge crowd that breached the airport perimeter.

Seven people reportedly died in total.

Some of those who reportedly died fell from an American military transport jet as it taxied for departure from the single runway.

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.