Five people on board a Russian military helicopter were killed today when it was shot down over Syria, in the single deadliest incident for Moscow since it intervened in the war.

The attack came as Syrian opposition fighters and their jihadist allies battled government forces outside Aleppo in a bid to ease the regime's siege of rebel-held parts of the northern city.

Russias defence ministry announced the downing of the helicopter, which it said was carrying three crew and two officers.

A Russian Mi-8 military transport helicopter was shot down from the ground after delivering humanitarian aid to Aleppo, the defence ministry said in a statement quoted by Russian news agencies.

The Kremlin said all five people on board were assumed dead.

It was not immediately clear who was responsible.

The incident was the deadliest single attack on Russian forces in Syria since Moscow began its intervention in support of President Bashar al-Assads government last September.

It brought the total number of members of the Russian forces killed in Syria to 18.