NATO forces in Afghanistan were less than honest over the killing of a prominent Afghan journalist by an American soldier, an investigation by a Kabul-based think tank charged Wednesday.

For several weeks NATO's International Security Assistance Force (ISAF) refused to even admit that US troops had been at the scene of the reporter's death, said a report by the Afghanistan Analysts' Network (AAN).

Journalist Omaid Khpulwak, who worked for the BBC and Afghan media, was killed on July 28 last year in a counter-attack when the offices of national broadcaster RTA in southern Uruzgan province were targeted by Taliban insurgents.

ISAF said the counter-attack was led by Afghan security forces, while the Afghan government blamed Khpulwak's death on the Taliban, the report by the AAN's Kate Clark says.

But in September, the US military published a summary of its investigation into the journalist's death, revealing that one of the US soldiers who cleared the building had shot the bearded Khpulwak after mistaking him for an insurgent.

Clark noted that although Afghan commandos did fight the Taliban at a separate attack on the governor's compound, "their role at RTA was dismal, virtually negligible".

"Yet the ISAF press release on the twin attacks gave a glutinously adulatory account of the Afghan forces and failed to mention any international military involvement.

"ISAF spokesmen have continued to try to spin the story -- claiming even recently that the counter-attack had been 'Afghan-led', when in fact no Afghans were involved at all," AAN said.

The report says Khpulwak's family received death threats in anonymous phone calls in which they were told to stop voicing their suspicions that a US soldier had killed him.

"They risked a great deal to give interviews and pass on ballistics and other evidence," said Clark.

"ISAF's failure to talk frankly with the media and Afghan population caused needless distress to Omaid's family and friends and helped spark suspicions of a cover up," she said.

Clark's report -- "Death of an Uruzgan journalist: Command Errors and Collateral Damage" -- acknowledges the efforts made by ISAF commander General John Allen to reduce civilian casualties caused by soldiers under his command, but says this must be allied to honesty when operations go wrong.

"We stand by our reporting on this unfortunate incident," a spokesman for ISAF, Lt-Col Jimmie Cummings, told AFP in response to the AAN investigation.

"Following this incident in July, ISAF was in communication with the family as well as BBC and they were also the first to be briefed on the findings of the report by ISAF on 8 September 2011.

"After a thorough investigation it was determined that Mr Khpulwak was killed in a case of mistaken identity.

"The reporter was shot by a US member who believed he was an insurgent that posed a threat and was about to detonate a suicide vest improvised explosive device.

"After thoroughly looking at all the evidence it was determined that US forces acted appropriately and within the ROE (rules of engagement) under the circumstances," Cummings said.