An initial review into the deaths of a Royal Marine and an army medic in Afghanistan has revealed the killings were not caused by “friendly fire”, the Ministry of Defence has said.

Corporal Channing Day, 25, who served with the 3 Medical Regiment, and Corporal David O’Connor, 27, of 40 Commando, were fatally injured while on patrol in the Nahr-e Saraj district of Helmand province on Wednesday.

An Afghan man, who is believed to have been a member of the Afghan Uniformed Police but who was not wearing uniform at the time, also died during the incident, an MOD spokesperson confirmed.

Cpl O’Connor, from Havant in Hampshire, and Cpl Day, from Comber in County Down, came under fire near the village of Char Kutsa, in the Nahr-e Saraj district.

The MoD says its analysis of the deaths is still continuing and “further investigation into the involvement or otherwise of the dead Afghan male is ongoing.”

British defense ministry officials said, “A joint incident assessment team, comprising specialists from International Security Assistance Force (Isaf), along with representatives from the government of Afghanistan, have conducted an initial review at the scene and have concluded that this was not a blue-on-blue [“friendly fire”] incident, and that the deaths of Corporal O’Connor and Corporal Day were caused by a third party or parties whose identities have yet to be established but who are not UK personnel.

“Further investigation into the involvement or otherwise of the dead Afghan male is ongoing”, the officials said adding that analysis of the event was “likely to take some time” while forensic examinations were carried out. A parallel Royal Military Police investigation was also being undertaken.

Defence Secretary Philip Hammond said the deaths were “a terrible tragedy” and it was still unclear what exactly had happened.

“We know there were three people dead, one of them wearing civilian clothes,” he said.

Asked if it was another so-called green on blue attack – where Afghans turn on their Isaf partners – Mr Hammond said: “There is a possibility that that is what has happened but it is not clear at this stage who the shooter actually was.”