The seven were held after a Taliban fighter was killed in Helmand province last year, the Sun reported.
All were taken into custody by military police and the Service Justice System will investigate.
The ministry said no civilians were involved in the incident.
It, however, refused to reveal whether the Taliban man was in custody when he was killed.
”The arrests relate to an incident in Afghanistan in 2011. It followed an engagement with an insurgent. There were no civilians involved,” a ministry spokesman was quoted as saying.
The ministry also did not name the arrested marines.
