Unidentified assailants shot dead seven members of Syria's White Helmets rescue service early Saturday during a raid on their base in a jihadist-held northwestern town, the group said.
The attackers struck in the town of Sarmin, nine kilometres (six miles) east of the city of Idlib, that is controlled by the Hayat Tahrir al-Sham jihadist alliance.
"The civil defence centre in Sarmin was the target of an armed attack by unknown assailants in which seven volunteers were killed," the White Helmets said in statement.
"Two minibuses, some white helmets and walkie-talkies were stolen."
It was not immediately clear whether the motives for the raid were political or purely criminal.
The Syrian Observatory for Human Rights said the seven volunteers had all been killed by bullets to the head.
"Colleagues came in the morning for the change of shift and found them dead," its director, Rami Abdel Rahman, told AFP.
The White Helmets emerged in 2013, working to rescue civilians in rebel-held areas.
They have since gained international renown for their daring rescues, often filmed and circulated on social media, and were nominated for the 2016 Nobel Peace Prize.
Although they work exclusively in rebel-held areas, they insist they are non-partisan.
Their detractors, including President Bashar al-Assad's government and his ally Russia, accuse them of being tools of their international donors.
They receive funding from a number of Western governments, including Britain, Germany, the Netherlands and the United States.
Critics also accuse them of harbouring rebel fighters, including jihadists, in their ranks.