British special forces operatives serving in the SAS repeatedly killed detainees and unarmed men in suspicious circumstances in Afghanistan, according to a new BBC investigation.

Military reports obtained by the British public broadcaster suggest that one unit from the elite British Special Air Service may have unlawfully killed as many as 54 people during one six-month tour.

The UK's defense ministry responded to the BBC's accusations by saying British troops had "served with courage and professionalism in Afghanistan."

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The BBC Panorama investigation saw journalists go through hundreds of pages of SAS operational accounts.

They include reports covering more than a dozen "kill or capture" raids executed by one SAS squadron in Helmand between 2010 and 2011.

Several individuals who served with the squadron told the BBC they witnessed the SAS operatives kill unarmed people during night raids.

They also accused operatives of planting AK47 rifles at a scene to justify the killing of an unarmed person.

Witnesses also told Panorama that SAS squadrons would compete with each other to kill the most people in the raids, and that the squadron scrutinised by the BBC was trying to hit a higher body count than the one it had replaced.

The Ministry of Defense told the BBC it could not comment on specific allegations, but that did not mean it accepted the allegations' factual accuracy. 

The Guardian newspaper quoted the MoD as saying investigations by military police into the allegations had resulted in no prosecutions. 

An MoD statement read: "Neither investigation found sufficient evidence to prosecute. Insinuating otherwise is irresponsible, incorrect and puts our brave armed forces personnel at risk, both in the field and reputationally."