A Syrian rebel-run first responder service says an airstrike has hit its training center in northern Syria, killing five personnel.

The Syrian Civil Defense, which conducts search and rescue operations in rebel-held areas, says the attack took place in the town of Atareb, west of the city of Aleppo, on Tuesday morning.

The organization called the attack a "massacre," on its social media pages.

The organization, also known as the White Helmets, has received global praise for its rescue operations in Syria.

The Britain-based activist group Syrian Observatory for Human Rights has confirmed the strike. The Observatory, a monitoring group that relies on a network of activists inside Syria, also reported that separately from the attack on White Helmets, pro-government forces also launched two rockets into Atareb on Tuesday.