He said that the arrested Marines have "very much let the side down."
Past incidents involving abuses by coalition soldiers have sparked protests and riots in Afghanistan, damaging relations with the government of Hamid Karzai. Also eroding the relations have been a rising tide of attacks in which Afghan soldiers or police assault their international allies — endangering the partnership that is critical to training Afghan security forces and withdrawing international troops.
But while there have been some allegations of abuse against civilians, the case revealed Thursday is unusual in that it involves an insurgent.
However, allegations of British misconduct in general have been rare in Afghanistan, with some of the experts noting that Britain had learned its lessons from several appalling incidents in Iraq.
Britain's six-year military presence in southern Iraq spawned multiple allegations of torture and abuse. 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 defense authorities later apologized for the mistreatment of Mousa and nine other Iraqis and paid a $4.8 million (3 million pound) settlement. Six soldiers were cleared of wrongdoing at a court martial, while another pleaded guilty and served a year in jail.
The government says abuse was committed by only a few soldiers, but lawyers for the alleged victims say it was systemic.
The Defense Ministry's terse statement Thursday suggested that the military was moving to control the incident, and trying to make certain the potentially lurid details were kept under wraps. The statement said the seven were arrested Thursday by Royal Military Police, but did not name the Marines.
While the ministry said the Marines were not arrested in Afghanistan, it would not specify where the arrests took place or give any further details on the alleged murder.
"The investigation will now be taken forward and dealt with by the Service Justice system," the ministry said in a statement, adding that "as with any serious incident of this nature, there will be an internal review to identify lessons learned."
Britain has 9,500 troops in Afghanistan — the second largest foreign force after the United States — based in the southern Helmand province.
About 500 troops will leave this year, ahead of the withdrawal of all international forces at the end of 2014.
Associated Press Writers Deb Riechmann in Kabul and Mirwais Khan in Kandahar, Afghanistan, contributed to this report.