The United States said it was up to Afghan security forces to defend the country after Taliban militants captured a sixth provincial capital on Monday, along with border towns and trade routes.

President Joe Biden has said the US military mission in Afghanistan will end on August 31, arguing that the Afghan people must decide their own future and that he would not consign another generation of Americans to the 20-year war.

US envoy for Afghanistan Zalmay Khalilzad has left for Qatar where he will "press the Taliban to stop their military offensive and to negotiate a political settlement", the State Department said on Monday.

In talks over three days, representatives from governments and multilateral organisations will press for "a reduction of violence and ceasefire and a commitment not to recognise a government imposed by force", the State Department said.

The Afghan Taliban, fighting to reimpose their rule after their 2001 ouster, have stepped up their campaign to defeat the government as foreign forces withdraw.

On Monday, they took Aybak, capital of the northern province of Samangan.

"Right now the Taliban are fighting with Afghan forces to capture the police headquarters and compound of the provincial governor," said Ziauddin Zia, a lawmaker in Aybak.

"Several parts of the capital have fallen to the Taliban."

The insurgents took three provincial capitals over the weekend — Zaranj in the southern province of Nimroz, Sar-e-Pul, in the northern province of the same name, and Taloqan, in northeastern Takhar province.

They had already taken the northern provincial capital of Kunduz and Lashkar Gah, the capital of Helmand province.

Pentagon spokesman John Kirby said the United States was deeply concerned about the trend but that Afghan security forces had the capability to fight the insurgent group.

"These are their military forces, these are their provincial capitals, their people to defend and it's really going to come down to the leadership that they're willing to exude here at this particular moment," Kirby said.

Asked what the US military can do if the Afghan security forces are not putting up a fight, Kirby said: "Not much."

US officials, speaking on condition of anonymity, said that while the military had warned Biden earlier this year that provincial capitals would fall with a withdrawal of troops, they were still surprised at how quickly some of them were being taken by the Taliban.

The United States carried out less than a dozen strikes over the weekend as the Taliban overran the provincial capitals, in one instance simply destroying equipment.

One official said the Afghan forces did not ask for any support as Kunduz was being overtaken.

Recriminations

The Taliban gains have sparked recriminations over the withdrawal of foreign forces. British Defence Secretary Ben Wallace told the Daily Mail that the accord struck last year between the United States and the Taliban was a "rotten deal".

Washington agreed to withdraw in a deal negotiated last year under Biden's Republican predecessor, Donald Trump.

Wallace said his government had asked some Nato allies to keep their troops in Afghanistan once the US troops departed, but failed to garner enough support.

"Some said they were keen, but their parliaments weren’t. It became apparent pretty quickly that without the United States as the framework nation it had been, these options were closed off," Wallace said.

Germany's defence minister rejected calls for its soldiers to return to Afghanistan after Taliban insurgents took Kunduz where German troops were deployed for a decade.

Afghan commandoes had launched a counterattack to try to beat back Taliban fighters who overran Kunduz, with residents fleeing the conflict describing the almost constant sound of gunfire and explosions.

In the west, near the border with Iran, security officials said heavy fighting was underway on the outskirts of Herat. Arif Jalali, head of Herat Zonal Hospital, said 36 people had been killed and 220 wounded over the past 11 days. More than half of the wounded were civilians.

Unicef said 20 children were killed and that 130 children had been injured in southern Kandahar province in the past 72 hours.

"The atrocities grow higher by the day," said Hervé Ludovic De Lys, Unicef's representative in Afghanistan.

Families flee

In Kunduz, many desperate families, some with young children and pregnant women, abandoned their homes, hoping to reach the relative safety of Kabul, 315 kilometres to the south — a drive that would normally take around 10 hours.

Ghulam Rasool, an engineer, was trying to hire a bus to get his family to the capital as the sound of gunfire reverberated through the streets of his hometown.

"We may just be forced to walk till Kabul, but we are not sure if we could be killed on the way. [...] Ground clashes were not just stopping even for 10 minutes," Rasool told Reuters.

He and several other residents, and a security official, said Afghan commandoes had launched an operation to clear the insurgents from Kunduz.

In Kabul itself, suspected Taliban fighters killed an Afghan radio station manager, government officials said, the latest in a long line of attacks targeting media workers.

Thousands were trying to enter Kabul, even after the city has witnessed attacks in diplomatic districts.

Speaking to Al Jazeera TV on Sunday, Taliban spokesman Muhammad Naeem Wardak warned the United States against further intervention to support government forces.