Unidentified assailants shot dead seven volunteers of Syria's White Helmets rescue service early Saturday during a raid on their base office in a northwestern town, the organisation said.
It was active in rebel-held eastern Aleppo before the territory was seized by forces loyal to President Bashar al Assad.
They receive funding from a number of Western governments, including Britain, Germany, the Netherlands and the United States.
The attackers also stole two vans, helmets and walkie-talkies, according to a statement from the group, which is formally known as the Syrian Civil Defense.
The observatory's director, Rami Abdel Rahman, said: "Colleagues came in the morning for the change of shift and found them dead".
They have since gained worldwide renown for their daring rescues, often filmed and circulated on social media, and were nominated for the 2016 Nobel Peace Prize.
Although its volunteers work in exclusively rebel-held areas, the group insists it is not partisan.
Their detractors, including President Bashar al Assad's regime and his ally Russian Federation, accuse them of being tools of their worldwide donors.
Critics have also claimed rebel fighters, including jihadists, operate in their ranks.
In this photo provided by the Syrian Civil Defense group known as the White Helmets, Syrian Civil Defense workers search through the rubble after airstrikes in the village of Hass in the Idlib province, Syria, Oct. 26, 2016.