Five of Britain's elite Royal Marines have been charged with murder "in relation to an incident which is alleged to have taken place in Afghanistan in 2011", the defence ministry says.
The five are among nine marines arrested - seven on Thursday and two in the past 48 hours. Four have been released without charge.
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Officials have said the incident involved an "engagement with an insurgent" in Helmand, where the majority of Britain's 9,500 troops in Afghanistan are deployed. They say no civilians were involved.
The five Marines "have now been charged with murder and they remain in custody pending court proceedings", a ministry spokesman said on Sunday.
"Everybody serving in theatre knows the rules of engagement," Philip Hammond, the defence secretary, said, vowing that "any abuse will be dealt with".
The London-based Guardian newspaper quoted the spokesman as saying: "The Royal Military police has referred the cases of the remaining five Royal Marines to the independent Service Prosecuting Authority.
"Following direction from the SPA these marines have now been charged with murder and they remain in custody pending court proceedings. It would be inappropriate to comment further on this ongoing investigation."
During a six-month tour of duty in 2010, which lasted from April to October, seven servicemen from 3 Commando Brigade were killed in action, all from 42 Commando, the Guardian added.
Footage on laptop
The BBC and other outlets reported that the arrests stemmed from video footage found on the laptop of a British serviceman who had been arrested on an unrelated charge.
Even though the incident does not involve a civilian, the case could cause a backlash from Afghans and further erode efforts to provide political stability to Afghanistan.
The brigade believed to be involved in the incident was in the thick of the fighting with Taliban fighters during its deployment last year to Helmand, which lies in Afghanistan's south.
British troops operate under rules of engagement, largely derived from the Geneva Convention, that dictate under what circumstances they are allowed to open fire.
Experts say the military has been strict about enforcing the rules after a disastrous period in Iraq, where there were multiple allegations of torture and abuse by British troops.
The most notorious case involved a hotel receptionist, Baha Mousa, who died while in custody at a British base after being detained in a raid in Basra in September 2003.
Britain's defence authorities later apologised for the mistreatment of Mousa and nine other Iraqis and paid a $4.8m settlement.
Six soldiers were cleared of wrongdoing at a court martial, while another pleaded guilty and served a year in jail.
Also on Sunday, Philip Hammond, British defence secretary, said his nation will thousands of the 9,000 troops it contributes to the International Security Assistance Force, ISAF, in Afghanistan in early 2013.
"I'd expect it will be significant, which means thousands, not hundreds, but I would not expect it to be the majority of our forces out by the end of next year....We will have by the beginning of 2013 we'll have 9,000 troops there, and by the end of 2014 we'll have virtually none, so I would expect there to be a significant step at the end of 2013," said the Defence Secretary.