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

The latest significant blow was the loss of the capital of Helmand province where American, British and Australian forces fought some of the bloodiest battles in the past 20 years. Hundreds of foreign troops were killed in the province, which is also a major opium hub.

The insurgents have taken half of the country’s 34 provincial capitals in recent days, including its second- and third-largest cities, Herat and Kandahar. The Taliban now control more than two-thirds of the country just weeks before the US plans to withdraw its last troops.

While the capital of Kabul isn’t directly under threat yet, the losses and advances elsewhere further tighten the grip of a resurgent Taliban.

Noose tightens on Kabul

The latest US military intelligence assessment suggests Kabul could come under insurgent pressure within 30 days and that, if current trends hold, the Taliban could gain full control of the country within a few months.

In the south, the insurgents swept through the capitals of Zabul and Uruzgan provinces, in addition to Helmand’s.

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

Atta Jan Haqbayan, the provincial council chief in Zabul province, said the local capital of Qalat fell and that officials were in a nearby army camp preparing to leave.

Bismillah Jan Mohammad and Qudratullah Rahimi, lawmakers from Afghanistan’s southern Uruzgan province, said local officials surrendered Tirin Kot to the Taliban. Mohammad said the governor was heading to the airport to depart for Kabul.

In the country’s west, meanwhile, Fazil Haq Ehsan, head of the provincial council in Ghor province, said its capital Feroz Koh also fell to the insurgents.

With security rapidly deteriorating, the United States planned to send in 3000 troops to help evacuate some personnel from the US Embassy in Kabul.

Separately, Britain said about 600 troops would be deployed on a short-term basis to support British nationals leaving the country, and Canada is sending special forces to help evacuate its embassy.

Falling like dominos

Thousands of Afghans have fled their homes amid fears the Taliban would again impose a brutal, repressive government, all but eliminating women’s rights and conducting public executions.

Peace talks in Qatar remain stalled, though diplomats are still meeting, as the US, European and Asian nations warned that any government established by force would be rejected.

“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, as they pushed into the capital of Logar province, just 80 kilometres south of Kabul.

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

The onslaught represents a stunning collapse of Afghan forces after the United States spent nearly two decades and $US830 billion ($A1.1 trillion) trying to establish a functioning state.

US forces toppled the Taliban in the wake of the September 11 attacks, which al-Qaida planned and executed while being sheltered by the Taliban government.

-with AAP