Rescue workers in Syria have been protesting after seven volunteers from the Syrian Civil Defense Units, popularly known as the White Helmets, were shot dead in their sleep.

The White Helmets rescue civilians from the rubble following air strikes in rebel-held areas in war-torn Syria.

The victims were fatally shot in their heads at their base in Sarmin, in Idlib province, by unknown assailants on Saturday (12 August). The motive of the attack remains unknown.

This is not the first time that White Helmets volunteers, which according to the organisation number more than 3,300, have been killed during attacks.

However, the organisation said the Saturday assassination was the "first of its kind".

White Helmet volunteers held vigils across Syria on Sunday (13 August).

"Every one of you has a story with the wounded and you have given your blood to save others," Raed al-Saleh, head of the White Helmets, was quoted as saying by the Guardian. "I hold all those who claim leadership responsible. You must uncover the criminals who carried out this heinous crime."

The organisation said in a statement: "The unknown armed group assassinated all the volunteers who were part of the night shift crew with a bullet in the head of each of them inside the center, in addition to the theft of two vans, white helmets, and walkie talkie devices type Hitra, in addition to other equipment.

"As a result, the Syria Civil Defense appeals to all checkpoints in northern Syria to ensure that every vehicle of the SCD holds an official task marked with an official seal. And not to hesitate the detention of any vehicle of civil defense that does not carry an official task until its belonging to the Syrian Civil Defense is confirmed," it continued.

The US State Department has issued a statemented saying it was "saddened and horrified" following the attack. "These cowardly acts of masked men took the lives of civilian volunteers who work tirelessly as first responders in order to save lives in incredibly dangerous environments," it said.

"We want to send our condolences to the families of these heroes and hope they are able to seek justice for the loss of their loved ones at the hands of these criminals.‎"

Sarmeen is controlled by former al-Qaeda affiliate previously known as as Jabhat al-Nusra, and now as Hay'at Tahrir al-Sham (HTS).

The group also condemned the attack, describing it as an "ugly crime", according to the BBC.

Last December, the White Helmets rescue group called for the help of the international community to evacuate East Aleppo fearing its members were at risk of torture and executions "in the regime's detention centres."

The organisation became internationally famous after it was nominated for a Nobel peace prize in 2016.

Its work was featured in the 2016 Netflix documentary White Helmets.

War in Syria

Damascus Syria Assad rebels
2 February 2017: Members of the Syrian Civil Defence, known as the White Helmets, search for survivors amid the rubble of a building following a reported air strike on the rebel-held suburb of Qaboun surburb Msallam Abdalbaset/AFP

Syria descended into war in 2011 , when President Bashar al-Assad deployed government troops to quash anti-government demonstrations in several areas of the country.

Assad refused to step down as violence spread throughout the region.

Despite several ceasefires and the intervention of international coalitions, the conflict deepened and fostered the emergence of anti-Assad rebel groups.

The warring sides involved in the conflict are numerous. A US coalition backs some of the militia groups that want to overthrow Assad, including the Free Syrian Army and Syrian Democratic Forces.

On the other side, Russia, Iran and Hezbollah back Assad forces.

The Isis terror group fights both pro- and anti-Assad forces and aims to establish an Islamic caliphate throughout the occupied territories.

The conflict has resulted in the deaths of hundreds of thousands of people and the displacement of millions.