The Taliban completed their sweep of Afghanistan’s south yesterday, taking four more provincial capitals in a lightning offensive that brought them closer to Kabul just weeks before the US is set to end its two-decade war.

In 24 hours, the country’s second and third-largest cities – Herat in the west and Kandahar in the south – fell to the insurgents, as has the capital of the southern province of Helmand, where American, British and Nato forces fought some of the bloodiest battles of the conflict.

The blitz through the Taliban’s southern heartland means the insurgents now hold half of Afghanistan’s 34 provincial capitals and control more than two-thirds of the country.

The Western-backed government in the capital, Kabul, still holds a smattering of provinces in the centre and east, as well as the northern city of Mazar-i-Sharif.

While Kabul is not directly under threat yet, the resurgent Taliban were battling government forces in Logar province, about 80km from the capital.

The US military has estimated that Kabul could come under insurgent pressure within 30 days and that the Taliban could overrun the rest of the country within a few months. They have already taken over much of the north and west of the country.

In the south, insurgents swept through three provincial capitals yesterday.

Attaullah Afghan, the head of the provincial council in Helmand, said the Taliban captured Lashkar Gah following weeks of heavy fighting and raised their white flag over governmental buildings. He said that three army bases outside of the city remain under government control.

In Tirin Kot, the capital of the southern Uruzgan province, Taliban fighters paraded through a main square, driving a Humvee and a pickup seized from Afghan forces. Local officials confirmed that the Taliban also captured the capitals of Zabul province and Ghor.

With security rapidly deteriorating, the US planned to send in 3,000 troops to help evacuate some personnel from the US Embassy in Kabul. Britain and Canada are also sending forces to aid their evacuations. Germany is reducing its embassy staff to the “absolute minimum”.

Hundreds of thousands of Afghans have fled their homes amid fears the Taliban will return the country to the sort of brutal, repressive rule it imposed when it was last in power at the turn of the ­millennium.

At that time, the group all but eliminated women’s rights and conducted public executions as it imposed an unsparing version of Islamic law.

An early sign of such tactics came in Herat, where insurgents paraded two alleged looters through the streets yesterday with black makeup smeared on their faces.

There are also concerns that the fighting could plunge the country into civil war, which is what happened after the ­Soviets withdrew in 1989.

“We are worried. There is fighting everywhere in Afghanistan. The provinces are falling day by day,” said Ahmad Sakhi, a resident of Kabul. “The government should do something. The people are facing lots of problems.”

The UN refugee agency said nearly 250,000 Afghans have been forced to flee their homes since the end of May, and 80pc of those displaced are women and children. In all, the agency said, some 400,000 civilians have been displaced since the beginning of the year.

Peace talks in Qatar between the Taliban and the government remain stalled, though diplomats are still meeting, as the US, European and Asian nations warned that battlefield gains would not lead to political recognition.

“We demand an immediate end to attacks against cities, urge a political settlement, and warn that a government imposed by force will be a pariah state,” said Zalmay Khalilzad, the US envoy to the talks.

But the Taliban advance ­continued.

Hasibullah Stanikzai, the head of the Logar provincial council, said fighting was still under way inside Pul-e Alim, with government forces holding the police headquarters and other security facilities.

He spoke by phone from his office, and gunfire could be heard in the background. The Taliban, however, said they had captured the police headquarters and a nearby prison.

The onslaught represents a stunning collapse of Afghan forces after the United States spent nearly two decades and $830bn (€700bn) trying to establish a functioning state.