The raid occurred as the rescue workers were breaking their Ramadan fast before dawn
Five members of the Syrian White Helmets were killed by masked gunman as they broke their Ramadan fast on Saturday morning, the group said.
The rescue group said the assailants stormed their Al-Hader centre in the northern province of Aleppo, blindfolded the men and shot them, before making off with equipment.
Four of the volunteers were killed on the spot while a fifth later died in hospital, the group wrote on Twitter. Two other men were wounded and two more successfully fled the attack.
Founded in 2013, the White Helmets are a network of first responders who rescue wounded in the aftermath of airstrikes, shelling or blasts in rebel-held territory.
The Al-Hader centre lies in a part of Aleppo province controlled by Hayat Tahrir al-Sham (HTS), a rebel group whose main component was once al-Qaeda's affiliate in Syria.
"A horrible massacre at dawn," was how the White Helmets described the attack on Twitter.
"Last night during the breaking of the Ramadan fast, an armed gang attacked the Al Hader White Helmets center."
Ahmad Al-Hamish, who heads the centre, said the assailants "blindfolded the staff members who were on the night shift, and killed five of them".
"Two others were wounded and another two were able to flee. The attackers were masked and escaped after stealing some equipment and generators," he added.
Last night during the breaking of the Ramadan fast, an armed gang attacked the Al Hader White Helmets center (Southern Aleppo) and killed four volunteers and wounded three others. May they rest in peace. pic.twitter.com/aTjT23LyMZ
— The White Helmets (@SyriaCivilDef) May 26, 2018
It was unclear whether the attack was a robbery-gone-wrong or if the centre and its crew had been specifically targeted.
More than 200 White Helmets rescuers have been killed in Syria's seven-year war, usually in bombing raids or shelling on their centres.
While attacks like the one on Saturday are rare, they have happened before.
In August, seven White Helmets members were killed in a similar attack in the town of Sarmin, in neighbouring Idlib province.
Most of Idlib is held by HTS, as well as a part of Aleppo and the adjacent province of Hama.
Tensions are on the rise there, with a wave of intra-opposition assassinations and clashes leaving at least 20 rebels dead in 48 hours, according to the Syrian Observatory for Human Rights.
"You cannot separate the Al-Hader incident from the assassinations and other killings that have been happening more and more in recent weeks in areas under HTS control," said Observatory head Rami Abdel Rahman.
The population of Idlib province has swelled to more than two million people as a result of large-scale transfers of rebels and civilians from one-time opposition zones elsewhere in the country.
The killings come as the White Helmets are facing a "freeze" on funding from the United States, which is still reviewing over $200mn earmarked for stabilisation in Syria.