The United States could slow down its withdrawal from Afghanistan amid rapid battlefield gains by the Taliban , the Pentagon said.

It came as the United Nations warned the insurgents were making strong gains and had captured 50 districts, many neighbouring Afghan cities.

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

US president Joe Biden has promised all US troops will be out of Afghanistan by September, but in recent weeks officials had briefed that the pullout was ahead of schedule and could be completed by next month.

“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,” John Kirby, a Pentagon spokesman, said.

“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.”

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

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

“We are seeing mass surrenders of Afghan security forces,” Bilal Sarwary, a Kabul-based journalist, told the BBC. The Taliban have shared videos on its 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 UN special envoy.

Deborah Lyons told the UN Security Council the announcement earlier this year that foreign troops would withdraw sent a “seismic tremor” through Afghanistan.

“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suggesting that the Taliban are positioning themselves to try and take these capitals once foreign forces are fully withdrawn,” the former Canadian diplomat said.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 50km from Kunduz city, came a day after the Taliban had encircled the city.

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