A Russian transport helicopter was shot down in opposition rebel territory in northern Syria on Monday, killing all five military personnel on board, the Kremlin said.

This incident is considered the first deadliest 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 veer away from inhabited areas to avoid civilian deaths.

There was no immediate claim of responsibility for the attack.

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. The helicopter appears to have broken up as it crashed, its tail can be seen lying separately from the aircraft’s body in flames.

Peskov stressed Moscow’s unremitting will to fighting international terrorism “on all fronts” despite threats from ISIS.

On Sunday, ISIS posted a nine-minute video message on YouTube, calling on its members to carry out jihad or holy war in Russia.