Seven Royal Marines have been arrested on suspicion of murder, the Ministry of Defence (MoD) says.
The arrests by the Royal Military Police related to an incident in Afghanistan last year, the BBC reports.
The MoD said the incident followed an "engagement with an insurgent" when 3 Commando Brigade was based in Helmand, and no civilians were involved.
It is thought to be the first time UK servicemen have been arrested on such charges during the conflict.
An MoD spokesman says: "The investigation will now be taken forward and dealt with by the service justice system.
"These arrests demonstrate the Department and the Armed Forces' determination to ensure UK personnel act in accordance with their rules of engagement and our standards.
"It would be inappropriate to make any further comment while the investigation is under way."
The Afghan Defence Ministry welcomed the announcement and spokesman General Zahir Azimi says it is important British and Nato forces respect the laws of war.
The armed forces' rules of engagement stipulate under what circumstances British forces are allowed to open fire, either in direct contact with the enemy or to prevent an imminent attack.