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

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

Russia's 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 ministry said in a statement quoted by Russian news agencies.

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

"As far as we know from the information we've had from the defence ministry, those in the helicopter died, they died heroically because they were trying to move the aircraft away to minimise victims on the ground," Kremlin spokesman Dmitry Peskov told journalists.

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-Assad's government last September.

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

The Syrian Observatory for Human Rights, a Britain-based monitor, said the helicopter had come down along the administrative border between Idlib province in the north-west and neighbouring Aleppo.

Idlib is held almost entirely by a powerful coalition of Islamist and militant forces, including the former Al-Nusra Front, now known as the Fateh al-Sham Front after renouncing its status as Al-Qaeda's Syrian affiliate.

AGENCE FRANCE-PRESSE