America could slow down its withdrawal from Afghanistan amid rapid battlefield gains by the Taliban which have raised alarm in Nato capitals, the Pentagon said.

Ashraf Ghani’s forces have been swept out of many rural areas since the insurgents launched a nationwide offensive at the start of May.

Joe Biden has promised all US troops will be out of Afghanistan by September, but in recent weeks officials had briefed that the pull out was ahead of schedule and could be complete as early as July.

John Kirby, Pentagon spokesman, said September remained the deadline and the pace could be adjusted by conditions.

"The situation in Afghanistan changes as the Taliban continue to conduct these attacks and to raid district centres as well as the violence, which is still too high," he told reporters.

Afghan security forces have reportedly been surrendering in large number

Credit: Reuters

“If there needs to be changes made to the pace, or to the scope and scale of the retrograde, on any given day or in any given week, we want to maintain the flexibility to do that," he said.

"We’re constantly taking a look at this, every single day: what’s the situation on the ground, what capabilities do we have, what additional resources do we need to move out of Afghanistan and at what pace."

"All of these decisions are literally being made in real time," he added.

Afghanistan’s main border crossing with Tajikistan was on Tuesday one of the Taliban’s latest gains.

 We are seeing mass surrenders of Afghan security forces," Kabul-based journalist Bilal Sarwary told the BBC. The Taliban have shared videos on their WhatsApp channels and websites showing government soldiers surrendering and being told to go home. 

The onslaught had taken 50 of 370 districts in Afghanistan since May according to the United Nations special envoy.

Deborah Lyons told the U.N. Security Council that the announcement earlier this year that foreign troops would withdraw sent a "seismic tremor" through Afghanistan.

Shir Khan Bandar (Afghanistan) — Map

"Those districts that have been taken surround provincial capitals, suggesting that the Taliban are positioning themselves to try and take these capitals once foreign forces are fully withdrawn," the former Canadian diplomat said.

The seizure of Shir Khan Bandar, about 30 miles from Kunduz city, came a day after the Taliban had encircled the city.

"Unfortunately this morning and after an hour of fighting the Taliban captured Shir Khan port and the town and all the border check posts with Tajikistan," said Kunduz provincial council member Khaliddin Hakmi.

Separately, an army officer told AFP: "We were forced to leave all check posts… and some of our soldiers crossed the border into Tajikistan."

America has already turned over several of its last remaining bases to the Afghan forces and generals say more than half of the last stage of withdrawal is complete.

Mr Kirby said US forces continued to support Afghan troops in fighting the Taliban, but that would soon no longer be possible.

"So long as we have the capability in Afghanistan, we will continue to provide assistance to Afghan forces," he said

"But as the retrograde gets closer to completion, those capabilities will wane and will no longer be available."