A Russian military helicopter was shot down over Syria on Monday, killing all five people on board in the single deadliest incident for Moscow since it intervened in the war.

The attack came as militants and their allies battled government forces outside Aleppo in a bid to ease the government'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 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.

Lieutenant General Sergei Rudskoy called the downing a “terrorist act” and said the defence ministry was still trying to confirm the fate of the Russian servicemen “through all possible channels”.

It was not immediately clear who was responsible.