Five Syrian rescue workers were killed in an attack by masked assailants Saturday on one of their centers in the northern province of Aleppo, the White Helmets said.
The rescue force said armed men stormed its Al-Hader center in a pre-dawn attack and fired on the first responders inside.
Four volunteers were killed on the spot and a fifth died later in hospital, it wrote on Twitter.
Founded in 2013, the White Helmets are a network of first responders who rescue wounded in the aftermath of air strikes, shelling or blasts in rebel-held territory.
The Al-Hader center lies in an area controlled by Hayat Tahrir al-Sham (HTS), a jihadist organization whose main component was once Al-Qaeda's affiliate in Syria.
"At around 2:00 am, an armed group stormed the Al-Hader center, blindfolded the staff members who were on the night shift, and killed five of them," said Ahmad al-Hamish, who heads the center.
"Two others were wounded and another two were able to flee. The attackers were masked and escaped after stealing some equipment and generators," he said.
It was unclear whether the attack was a robbery-gone-wrong or if the center 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 centers.
While attacks like the one on Saturday are rare, they have happened before. In August, seven White Helmets members were killed in a similar assault in the town of Sarmin, in neighboring Idlib province.
Most of Idlib is held by HTS, as well as a part of Aleppo and the adjacent province of Hama.