Such killings of captured troops have been a common practice of the Islamic State in Iraq and the Levent group. However al-Qaeda's branch in Syria, known as the Nusra Front, has not carried out such mass killings in the past.
The so-called Syrian Observatory for Human Rights said 56 soldiers were executed in the Abu Zuhour air base that was captured by the Nusra Front and other militant groups on Sept. 9.
Mohammed Kanaan, an activist based in Syria, said the Nusra Front murdered 45 soldiers, adding that the executions took place Friday on a runway in the air base, The Associated Press reportd.
The UK-based group said the latest killings raise to 71 the number of Syrian army that have been killed since the capture of the air base by the militants.
More than 250,000 people have died in Syria's civil war, according to UN officials.