Seven civil defense volunteers were shot dead in north-eastern Syria on Saturday, according to a Syrian paramedic group.
News of the deaths was confirmed via the rescue service's Twitter feed.
The Syrian Civil Defense gathering, all the more prevalently known as the White Helmets, said in an announcement that the assault occurred in the early hours in the town of Sarmin.
"Two minibuses, some white helmets and walkie-talkies were stolen".
Hayat Tahrir al-Sham denounced what it called an "ugly crime" and vowed to track down the perpetrators and bring them to account.
No one immediately claimed responsibility for the rare attack but it came amid tension in the area. "It may likewise be an assault expected to hurt the picture of the Nusra Front and to demonstrate that Idlib is not protected", said Rami Abdurrahman who heads the Observatory.
They operate a rescue service in rebel-held parts of Syria, which have been subjected to fierce bombardment by the government and Russia's air force that has levelled whole city districts during the country's civil war.
They have since gained widespread global renown for their daring rescues, with footage often taken and circulated on social media.
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Although they work exclusively in rebel-held areas, they are non-partisan.
Their detractors, including President Bashar al-Assad's government and his ally Russian Federation, accuse them of being tools of their global donors.
The organization stresses its neutrality and not supporting ay political or armed group - it receives funds from UK, Netherlands, Denmark, Germany, Japan and the US.
Critics also accuse them of harbouring rebel fighters, including Islamist militants, in their ranks.
Seven of the rescue workers were massacred in an attack which has yet to be claimed by any group.
Abu Kifah was captured on camera crying as he pulled a baby girl alive from the rubble of a bomb blast.
The organization was nominated for the Noble Peace Prize in 2016 but didn't win - it commenced operations in 2013 and the volunteers have been known since 2014 as the White Helmets for the sake of the helmets they put on their heads.
Last year, it was the subject of an Oscar-winning documentary which showed the group's heroic efforts in desperate and unsafe situations.