The Syrian Observatory for Human Rights said military intelligence forces executed the prisoners who were being held in a makeshift facility in the city in northwestern Idlib province.
“Military intelligence members executed 23 prisoners before their withdrawal from the area of the National Hospital in the southwest of Jisr al-Shughur,” the Observatory said.
Observatory director Rami Abdel Rahman said the prisoners were being held near the main hospital building.
News of the deaths came after a coalition of forces including Al-Qaeda’s Syrian affiliate Al-Nusra Front seized Jisr al-Shughur on Saturday morning.
Al-Nusra also reported the apparent executions on one of its official Twitter accounts, posting a graphic photo showing the bodies of at least 14 men in a concrete room, some on top of others.
The walls and floor of the room were smeared with blood, and bloodstains marked the shirts and trousers of several men.
“A massacre carried out by the Nusayri army near the National Hospital before their withdrawal from the city of Jisr al-Shughur,” the caption to the picture read, using a derogatory term for the Alawite sect to which President Bashar al-Assad belongs.
Last month, the Observatory and other sources reported a similar execution of prisoners by government forces fleeing Idlib’s provincial capital as rebels and Al-Nusra entered the city.
At least 15 bodies were found in Idlib city’s military intelligence detention facility after the provincial capital fell on March 28.