Five Syrian rescue workers were killed in an attack by masked assailants Saturday on one of their centres in the northern province of Aleppo, the White Helmets said.
The rescue force said armed men stormed its Al-Hader centre 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 centre lies in an area controlled by Hayat Tahrir al-Sham (HTS), a jihadist organisation whose main component was once Al-Qaeda's affiliate in Syria.
"At around 2:00 am, an armed group stormed the Al-Hader centre, blindfolded the staff members who were on the night shift, and killed five of them," said Ahmad al-Hamish, who heads the centre.
"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 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 assault 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.
- 5 dead in Idlib -
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 massive transfers of rebels and civilians from onetime opposition zones elsewhere in the country.
On Saturday, five people were killed in a blast in the urban capital of the province, also called Idlib, according to the Britain-based Observatory.
"Two were civilians, including a child. There was also one Uzbek fighter and two unidentified people," said Abdel Rahman.
The explosion hit a wide street in Idlib lined by tall cement apartment blocks. Several cars parked outside had been burned by the explosion.
White Helmets rescuers could be seen carrying several wounded out of the building, including a crying infant and a wounded man on a stretcher, to ambulances parked nearby.
The killings come as the White Helmets are facing a "freeze" on funding from the United States, which is still reviewing over $200 million earmarked for stabilisation in Syria.