(AFP) BEIRUT, Lebanon — 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 White Helmets said armed men stormed the 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 a part of Aleppo province 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 a.m., 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 attack 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.

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 2 million people as a result of massive transfers of rebels and civilians from onetime 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 $200 million earmarked for stabilization in Syria.