Seven civil defense volunteers were shot dead in north-eastern Syria on Saturday, according to a Syrian paramedic group.
It was active in rebel-held eastern Aleppo before the territory was seized by forces loyal to President Bashar al Assad.
"The attackers stole two ambulances, helmets and communication devices", Hossam Halaq told Anadolu Agency.
A spokesman for the Syrian Civil Defence group said gunmen stormed their camp in the town of Sarmin in Idlib province.
It was not immediately clear whether the raid was carried out for political or criminal motives.
International Youth Day 2017 observed globally
After the youth envoy invited everyone to join in mobilising young people, Guterres said, "Together, we can create a peaceful world for generations to come".
They receive funding from a number of Western governments, including Britain, Germany, the Netherlands and the United States.
The UK-based Syrian Observatory for Human Rights said the seven volunteers who died had all been shot in the head.
The White Helmets emerged in 2013, working to rescue civilians in many rebel-held areas.
President Bashar al Assad's government, meanwhile, accuses the group of faking videos and being a tool of its worldwide donors.
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.