LONDON (AFP) - 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 on Tuesday.

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

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 Sept 15, 2011 in the restive southern province of Helmand. The men, who were arrested last year, each denied 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, south-west 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 Taleban. 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 was 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.