A British marine executed an injured Afghan at close range while serving in Afghanistan in 2011, telling him "it's nothing you wouldn't do to us", a military court heard Tuesday.

The killing and the serviceman's comments were recorded by a camera mounted on the helmet of a co-defendant, which is thought to have turned on accidentally, prosecutors say.

Afterwards, the marine allegedly turned to his comrades and said: "Obviously this doesn't go anywhere fellas. I just broke the Geneva Convention."

Three Royal Marines, identified only as Marines A, B and C, have been charged with murdering an unknown captured Afghan national on or about September 15, 2011 in the restive southern province of Helmand.

The men, who were arrested last year, each deny the charge.

"It was not a killing in the heat and exercise of any armed conflict. The prosecution case is that it amounted to an execution, a field execution," prosecuting lawyer David Perry told the court martial hearing in Bulford, southwest England.

It was "an execution of a man who was entitled to be treated with dignity and respect and entitled to be treated as any British serviceman or servicewoman would be entitled to be treated in a similar situation", Perry said.

He told the court the victim was injured by fire from an Apache helicopter which had been searching for insurgents who attacked a command post near Forward Operating Base Shahzad, a base for international forces fighting the Taliban.

The marines were ordered to find out whether the Afghan was still alive, the court heard. They located him in an open field, seriously injured and armed with an old Kalashnikov, ammunition and a hand grenade.

It is claimed that Marine A told the others to move the man to a more secluded location, where he shot him in the chest at close range with a 9mm pistol.

Perry said the other two marines "were all party to the killing", claiming they "encouraged and assisted Marine A".

He accused the three servicemen of using the Afghan man's injuries as a "cloak" to conceal their own actions, saying they reported that he had died from the helicopter attack.

The recording, from a camera mounted on Marine B's helmet, was backed up by a journal written by Marine C which states he was encouraging Marine A to shoot, Perry said.

Prosecutors did not have the Afghan man's body, as it was left on the ground and was later removed by locals, he added.