A deadly explosion in Syria left 27 dead. Here, a young Syrian girl gets treated for injuries following airstrikes. Reuters

A deadly gas explosion at a clinic in Syria left 27 people dead in Hasakah, a northeast province. The fire was caused by the explosion of a gas canister at the Maysaloun clinic in Gamishli Sunday, and most of the victims were children.

Kurdish official Juan Mohammed said the clinic was full of children waiting for polio vaccines. The virus can lead to paralysis but is preventable with proper vaccinations.

The state news agency SANA said dry branches caught fire and spread to a nearby fuel tank.

"At least 27 people were killed ... and at least 30 people are being treated for burns," SANA said, citing a doctor who arrived at the scene to care for the injured.

"The fire took place east of the city at the Maysaloun clinic. We had just begun a vaccination campaign for the children with the help of the U.N.," an activist told Al Jazeera.

"The death toll could rise to 50, including nurses, women and children. We found 18 burned bodies. It was horrific. We have cases of second- and third-degree burns" he added. Many of the explosion’s victims suffered extreme burns.

At least 65 people are also dead following government airstrikes in Qamishli, including 22 civilians and 43 fighters from the Islamic State group. The group, also known as ISIS, launched a deadly assault on the city of Hasakah last week after clashing with pro-government National Defense Forces for control of the area. ISIS fighters also battled rival armed groups in Aleppo Sunday, seizing control of several towns near the Turkish border. An estimated 50 fighters belonging to the terror group have been reported dead in the past two day of conflict.