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, where 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.

He said the UK patrol was not working with any Afghan partners at the time.

He said: "A Joint Incident Assessment Team, comprising specialists from 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 Cpl O'Connor and Cpl 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."

He said analysis of the event is "likely to take some time" while forensic examinations are carried out. A parallel Royal Military Police investigation is also being undertaken.

Defence Secretary Philip Hammond paid tribute to Cpl O'Connor and Cpl Day and said what happened remains unclear.

He said: "It is a terrible tragedy in Afghanistan and I would like to send my heartfelt condolences to the family and friends of the soldier and marine who were killed.

"The circumstances are not entirely clear, there is an investigation ongoing. We know there were three people dead, one of them wearing civilian clothes."