The Taliban is intensifying a search for people who worked with US and NATO forces, a confidential United Nations document says, despite the militants vowing no revenge against opponents.

The report -- provided by the UN's threat-assessment consultants and seen by AFP -- says the group has "priority lists" of individuals it wants to arrest.

Most at risk are people who had central roles in the Afghan military, police and intelligence units, according to the document.

The Taliban have been conducting "targeted door-to-door visits" of individuals they want to apprehend and their family members, the report says.

It adds that militants are also screening individuals on the way to Kabul airport and have set up checkpoints in major cities, including the capital and Jalalabad.

The document, dated Wednesday, was written by the Norwegian Center for Global Analyses, an organization that provides intelligence to UN agencies.

"They are targeting the families of those who refuse to give themselves up, and prosecuting and punishing their families 'according to Sharia law,'" Christian Nellemann, the group's executive director, told AFP.

"We expect both individuals previously working with NATO/US forces and their allies, alongside with their family members to be exposed to torture and executions.

"This will further jeopardize western intelligence services, their networks, methods and ability to counter both the Taliban, ISIS and other terrorist threats ahead," he added.

Taliban spokesman Zabihullah Mujahid (left) at a press conference in a building where the Afghan government used to brief media
Taliban spokesman Zabihullah Mujahid (left) at a press conference in a building where the Afghan government used to brief media AFP / Hoshang HASHIMI

The report says the militants are "rapidly recruiting" new informers to collaborate with the Taliban regime and are expanding their lists of targets by contacting mosques and money brokers.

It reprints a letter, dated August 16, from the Taliban to an individual who worked in counter-terrorism in the Afghan government.

The letter asks the person to report to Taliban officials to "provide information about the nature of your work and relationship with the British and Americans."

"If you do not report to the commission, your family members will be arrested instead, and you are responsible for this. You and your family members will be treated based on Sharia law," it says.

The Norwegian Center for Global Analyses also warned the Taliban may target or arrest remaining Westerners or other foreign personnel, including medical workers, if they criticize the militants.

A UN spokesman did not respond to request for comment on the document.

The Taliban have launched a public relations blitz since sweeping back into power on Sunday, completing a stunning rout of government forces as the United States and other foreign troops withdrew following a 20-year occupation.

Among promises such as rights for women and an inclusive government, the militants have also pledged full amnesty for all who worked with the Western-backed elected Afghan government.

But Afghans have not forgotten the Taliban's ultra-conservative Islamic regime of 1996-2001, when brutal punishments, such as stoning to death for adultery, were imposed.

Tens of thousands of people have tried to flee the country since the Taliban takeover, sparking chaos at Kabul airport.