Five crew members have been killed as a Russian military chopper was shot down in Syria, the Kremlin said.

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The helicopter shot down on Sunday in the rebel-held Idlib province had three crew members and two officers on board.

Russia's defence ministry was cited as saying that the helicopter, an Mi-8 military transport helicopter, had been shot down after delivering humanitarian aid to the city of Aleppo as it made its way back to Russia's main airbase in Syria.

"There were three crew members and two officers from the Russian reconciliation centre in Syria on board," the ministry said in a statement. 

Kremlin spokesman Dmitry Peskov expressed condolences over the deaths of the five soldiers. 

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 "As far as we know from the information we've had from the defence ministry, those in the helicopter died," he said.

Russia is backing the Syrian government in the civil war with military support.

Photos had circulated on social media of the "smoking wreckage of the helicopter in the deserts of Syria after it was gunned down", said Al Jazeera's Rory Challands.

"There are [on the images] lots of people standing around this wreckage and the body of a man being dragged away with various bits of documentation and ID being displayed," he said.

"The signs were there that this would be something very sorry the Russian government would want to admit to."

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It was not immediately clear who was responsible.

Mamoun Abu Nuwar, an Amman-based Jordanian military analyst, said Russia is being "very careful" about releasing information on the incident.

"I think it's been shot down by small fire arms or anti-aircraft guns," he told Al Jazeera, explaining that the Army of Conquest, an alliance of armed groups, is most likely to have shot down the aircraft in that region. 

"Possibly we'll see some punishment by the Russians on these people in that area - conducting or escalating an air power campaign in the area."

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.

Tuesday's deaths brought the total number of 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 northwest and neighbouring Aleppo.

Idlib is held almost entirely by a powerful coalition of hardline rebel groups, including the former al-Nusra Front, now known as the Fateh al-Sham group after renouncing its status as al-Qaeda's Syrian affiliate.

Source: Al Jazeera and agencies