Seven members of Syria’s White Helmets rescue service - also known as the Syrian Civil Defence - were shot dead during a raid on their base in a militant-stronghold.
The volunteers were attacked on Saturday morning in Sarmin, six miles east of the city of Idlib, which is controlled by the Hayat Tahrir al-Sham jihadist alliance. All seven had been shot in the head by "unidentified assailants"
News of the deaths was confirmed via the rescue service’s Twitter feed.
The White Helmets later said in a statement: "The civil defence centre in Sarmin was the target of an armed attack by unknown assailants in which seven volunteers were killed.
"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.
Hayat Tahrir al-Sham denounced what it called an "ugly crime" and vowed to track down the perpetrators and bring them to account.
The militant group said the attack was aimed at "undermining our revolution", adding that it stood ready to protect the White Helmets in "liberated areas" of Syria.
The Syrian Observatory for Human Rights said the seven volunteers had all been killed by bullets to the head.
Its director, Rami Abdel Rahman, said: "Colleagues came in the morning for the change of shift and found them dead."
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.
Some reports have claimed one of the men killed in Saturday’s attack is a volunteer who featured in one of the group’s most moving rescues.
Abu Kifah was captured on camera crying as he pulled a baby girl alive from the rubble of a bomb blast. The footage was widely circulated on social media and was shown on news channels all over the world.
The attack came on the same day the White Helmets were awarded the Manhae Peace Prize in South Korea.
White Helmet volunteers are trained in Turkey to provide emergency medical assistance to civilians caught in Syria's civil war. They are often the first on the scene after bombings to help those injured and pull survivors from the rubble.
Last year, it was the subject of an Oscar-winning documentary which showed the group’s heroic efforts in desperate and dangerous situations.
The film, which was made by Netflix, received a standing ovation when it won the Academy Award for Best Documentary Short at this year’s Oscars.