An Afghan accused of torturing and murdering civilians while working for US special forces denies the charges and says he followed American orders, according to a report obtained by AFP on Tuesday.

Zakrya Kandahari, who worked as an interpreter, was arrested by Afghanistan's premier intelligence agency around six weeks ago.

According to a copy of an investigation report confirmed as authentic by a security official, intelligence agents have a video showing Kandahari beating a prisoner.

Afghan authorities are investigating allegations that armed Afghans working with US special forces harassed, tortured and murdered civilians in Wardak province, a Taliban flashpoint on the doorstep of Kabul.

President Hamid Karzai ordered US special forces to leave Wardak in February, although a compromise deal later announced that they would leave only Nerkh, one of eight districts in Wardak and the district where Kandahari worked.

"Zakrya himself has denied the accusations, saying he was under the command of others," said a copy of the report obtained by AFP.

Instead he blamed the murders on three Americans, whom he named as Dave, Hagen and Chris, and whom he said were fluent in Afghanistan's two main languages Dari and Pashto.

"'I was simply a low-rank translator and did not have authorisation to roam around inside the base, or (go) to interrogation sections," the report quoted Kandahari as saying.

Kandahari told interrogators that he had worked for foreigners for nine years, most recently as an interpreter for US special forces in Nerkh.

Though he denies torturing, he did admit to "slapping" and "kicking" the detainee allegedly shown in the video reportedly obtained by Afghan intelligence agents, according to the report.

He said "Dave" asked him to collect the suspect from the Afghan army in Nerkh.

"I went to their office, took the prisoner who was speaking rudely and slapped him several times in front of others and transferred him for further investigation to our own camp," the report quoted him as saying.

"I kicked him several times while taking him to the base and handed him over to Mr Dave and Mr Hagen at at around 5pm," it says.

He was quoted as saying that at around 9pm he saw the detainee's body "in a black plastic bag inside a tent" and that four other Afghan translators also witnessed that.

Zakrya was picked up in the southern city of Kandahar, the spiritual capital of the Taliban, and moved to Kabul for interrogation.

US investigations have found "no credible evidence" to substantiate any allegations of abuse by either NATO or US forces, a military spokesman told AFP on Tuesday.

But the military is co-operating with the Afghan government and US Army criminal investigators are also looking into the claims, said Lieutenant Colonel Will Griffin from the US-led NATO force.

"The allegations were taken very seriously and we just want to ensure that every possible outcome is thoroughly investigated," Griffin told AFP.