Russian copter downed

Five people on board a Russian military helicopter were killed yesterday 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.

Meanwhile, US warplanes yesterday carried out air strikes on positions of the Islamic State group in the Libyan city of Sirte for the first time, the country's unity government head announced.

"The first American air strikes on precise positions of the Daesh (IS) organisation were carried out today, causing heavy losses... in Sirte," prime minister Fayez al-Sarraj said in a televised speech.

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 defence 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.