MOSCOW — A Russian transport helicopter was shot down in opposition rebel territory in northern Syria on Monday and all five crew and officers onboard were killed, the Kremlin said, in the deadliest single incident for the Russian military since its entrance into Syria’s civil war.

The Mi-8 helicopter was shot down in Idlib province while returning to the Russian air base on Syria’s coast after delivering humanitarian goods to the city of Aleppo, the Defense Ministry said in a statement. The helicopter had three crew members and two officers deployed with the Russian center at the Hemeimeem air base on the Syrian coast.

“From what we know from information provided by the Defense Ministry, all those who were on the helicopter died,” Russian President Vladimir Putin’s spokesman told journalists. The ministry statement released earlier said their fate was still unknown.

Putin’s spokesman, Dmitry Peskov, said the Russians “died heroically because they tried to move the aircraft away so to minimize losses on the ground.”

There was no immediate claim of responsibility for the attack.

Idlib province has a strong presence of fighters both for the al-Qaida branch in Syria known as the Nusra Front and other groups fighting against Syrian President Bashar Assad’s forces. The Nusra Front announced last week that it was changing its name and relinquishing ties with al-Qaida in an attempt to undermine a potential U.S. and Russian air campaign against its fighters.

The group is part of a coalition of insurgent groups called Jaish al-Fateh, or Army of Conquest, which has captured most of Idlib.

Videos uploaded online by Syrian opposition activists show the burning wreckage of a Russian helicopter in footage seemingly taken in the first few moments after the helicopter crashed.

In one video, a rocket pod can be seen next to the wreckage. People standing nearby are seen taking cellphone photos and shouting “Allahu Akbar,” or God is great in Arabic.