Scores of patients, children and nurses were killed while dozens of others were injured in a gas explosion that took place in a clinic located in Syria's northeastern province of Hasakah, a monitoring group and activists said.

A fire caused by the explosion of a gas canister at the Maysaloun clinic in Qamishli on Sunday left at least 40 people killed and dozen others badly injured, activists told Al Jazeera.

"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 UN," a local activist who requested to remain anonymous said.

"The death toll could rise to 50, including nurses, women and children. We found 18 burnt bodies, it was horrific. We have cases of second and third degree burns" he added.

The UK-based Syrian Observatory for Human Rights said dozens of people were killed in the fire, and did not report a final death toll yet.

The Syrian Observatory said that many of those wounded are in critical conditions and suffered extreme burns, adding that the reason behind the explosion has not been determined.

The group also confirmed the death of at least 65 people overnight due to government air strikes on the city of Qamishli. The death toll includes 22 civilians and 43 fighters from the Islamic State of Iraq and the Levant (ISIL).

PHOTO CAPTION

Maysaloun clinic after the explosion [Local activists]

Aljazeera