Afghans detained domestically on terror or security charges are being routinely tortured, the United Nations said Wednesday, expressing particular concern over conditions in centres run by the country's spy agency.

The UN Assistance Mission in Afghanistan's "torture report" said its latest survey suggested a third of all Afghans detained by security officials reported being ill-treated.

The UNAMA report was based on interviews with more than 650 people suspected or convicted of security or terrorism-related offences in dozens of government facilities over a 15-month period up to March 2020.

The figure rose to almost 60 percent in the southern Kandahar province, the birthplace of the Islamist Taliban.

"It remains a matter of serious concern that more than 30 percent of all interviewees provided credible and reliable reports of torture and ill-treatment," the UNAMA report said. 

While overall allegations of abuse declined by three percent in 2019-2020 compared to the previous year, the rate of such treatment continues to be high, it said.

The report expressed particular concern with the practice of "solitary and incommunicado detention" in centres run by the country's spy agency, the National Directorate of Security. 

Afghan prisons are overcrowded with inmates often complaining of inadequate food, contaminated water and lack of medical supplies which regularly lead to the spread of contagious diseases.

Thousands of Taliban fighters are currently held in Afghan detention centres and jails, although about 5,000 were released ahead of peace talks.

Afghan forces are struggling to contain rising violence despite peace talks between the warring Taliban and Afghan government launching in September.