The deaths of a British marine and an army medic in Afghanistan were not due to "friendly fire", the defence ministry said Saturday following an initial review.

Corporal David O'Connor, 27, of 40 Commando Royal Marines, and Corporal Channing Day, 25, from the 3 Medical Regiment, were fatally wounded while on patrol in the southern Helmand province on Wednesday.

An Afghan man, thought to have been a policeman out of uniform, also died during the incident, a defence ministry spokesman confirmed.

He said the British patrol in the Nahr-e Saraj district was not working alongside Afghan partners at the time.

Specialists from the NATO-led International Security Assistance Force and representatives from the Afghan government have conducted an initial review at the scene.

They "have concluded that this was not 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," the spokesman said.

"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 conducted.

More than 50 NATO soldiers have been killed this year by men in Afghan uniform, leading to tensions between coalition troops and the local forces they are trying to train to take over security operations.

The deaths bring to 435 the number of British troops killed in the Afghanistan operation since it began in October 2001.