Mitch McConnell has warned that America’s retreat from Afghanistan risks a replay of the nation’s humiliating withdrawal from Saigon at the end of the Vietnam conflict in 1975.

As thousands of American soldiers were ordered to Afghanistan to evacuate embassy staff from Kabul amid a rapid advance by the Taliban, US Senate minority leader McConnell said the US was “careening toward a massive, predictable, and preventable disaster”.

It came as officials confirmed on Friday that the Taliban had captured Afghanistan’s second biggest city, Kandahar, as well as Lashkar Gah in the south.

The Taliban also claimed they had captured the western city of Herat, the country’s third-largest, and Qala-e-Naw in the north-west.

A photo that immortalised America’s defeat in Vietnam, showing evacuees boarding a helicopter on the roof of a building, spread fast on social networks after the United States announced the emergency deployment on Thursday.

McConnell, the most senior Republican in Congress, criticised the Biden administration on Thursday for its decision to announce the withdrawal of troops by 11 September – the 20th anniversary of the terror attack on New York and Washington that precipitated the US invasion of Afghanistan.

Although Biden’s predecessor, Donald Trump, had signalled the withdrawal, McConnell gave a searing assessment of the White House plan.

“The latest news of a further drawdown at our embassy and a hasty deployment of military forces seem like preparations for the fall of Kabul,” McConnell said.

“President [Joe] Biden’s decisions have us hurtling toward an even worse sequel to the humiliating fall of Saigon in 1975.

“President Biden is finding that the quickest way to end a war is to lose it,” McConnell said, urging the president instead to commit to providing more support to Afghan forces.

“Without it, al-Qaida and the Taliban may celebrate the 20th anniversary of the September 11 attacks by burning down our embassy in Kabul.”

A former US state department spokesperson, Morgan Ortagus, added her weight to the chorus of criticism, saying that it was “a huge foreign policy failure with generational ramifications just shy of seven months into this administration. Everything points to a complete collapse.”

Biden said on Tuesday that he does not regret his decision, noting that Washington has spent more than a trillion dollars in America’s longest war and lost thousands of troops. He added the United States continued to provide significant air support, food, equipment and salaries to Afghan forces.

Back in June, as the Taliban advance built momentum, Biden addressed the Saigon parallels and dismissed them out of hand. “There’s going to be no circumstance where you’ll see people being lifted off the roof of an embassy of the United States from Afghanistan,” he said.

The same month – since which the Taliban’s lightning offensive has surprised many US military officials – the chairman of the US joint chiefs of staff, Gen Mark Milley, also rejected comparisons to the desperate exit from Saigon.

“I do not see that unfolding,” Milley said. “I may be wrong, who knows, you can’t predict the future, but I don’t see Saigon 1975 in Afghanistan. The Taliban just aren’t the North Vietnamese army. It’s not that kind of situation.”

To carry out the evacuation of American staff from its embassy in Kabul, 3,000 US troops will secure the airport, 1,000 will be sent to Qatar for technical and logistical support, while 3,500 to 4,000 will be positioned in Kuwait to deploy if needed.

Senior US officials spoke to Afghanistan’s president, Ashraf Ghani, on Thursday and told him the US “remains invested in the security and stability of Afghanistan” in the face of Taliban violence, the state department said.

Antony Blinken, the US secretary of state, and Lloyd Austin, the defence secretary, told Ghani that Washington was reducing its civilian footprint in Kabul given the “evolving security situation” and would increase the tempo of special immigration visa flights for Afghans who helped the US effort in the country.

The UK said it would send 600 troops, and the defence secretary, Ben Wallace, said Britain was relocating its embassy from the outskirts of the secure Green Zone to a potentially safer location closer to the centre of the capital.

On Thursday, US officials scrambled to answer questions about the mission, with the Pentagon spokesman John Kirby declining to describe it as a so-called “noncombatant evacuation operation”, or NOE. He indicated it had no name, and avoided talking about evacuations.

The most famous “NOE” mission was Operation Frequent Wind, during which more than 7,000 Vietnamese civilians were evacuated from Saigon on 29-30 April 1975 by helicopter.

The image of American diplomats departing under military protection from the top of an apartment building used by the CIA – not the US embassy as often believed – has come to represent American failure in Indochina.

Asked about the image and the inevitable comparisons between the situation in Afghanistan and the fall of Saigon, Kirby tried to underline the differences.

“We are not completely eliminating our diplomatic presence on the ground,” he said.

“Nobody is abandoning Afghanistan, it’s not walking away from it. It’s doing the right thing at the right time to protect our people.”

The Taliban’s capture of Kandahar capped an eight-day blitz that has left only the capital and pockets of other territory in government hands. The group has established a bridgehead within 95 miles (150km) of Kabul and its rapid advances leave the capital isolated from the rest of the country.