BEIRUT, Lebanon — Islamist extremists in Syria’s insurgency killed 150 soldiers in a battle for control of an Aleppo suburb this week, including 51 who were executed after they had surrendered, according to a Syrian monitoring group that has been chronicling casualties and evidence of atrocities.

The group, the Britain-based Syrian Observatory for Human Rights, which sympathizes with the opposition and reports daily tallies of combat and violence from a network of contacts on the ground in Syria, corroborated its assertion on Friday with a video posted on the Internet.

The video, dated Wednesday, showed what appeared to be an execution ground, with dozens of lifeless bodies clumped against a wall pockmarked with bullet holes. The video’s authenticity could not be independently confirmed, but Rami Abdulrahman, the Syrian Observatory founder, quoted witnesses as saying 51 soldiers taken prisoner were later shot. Among the total who had surrendered, he said, “very few were taken hostage.”

Mr. Abdulrahman said the killers were members of the Nusra Front and Ansar al-Khalafa al-Islamiya Brigade, among the many jihadist groups, some affiliated with Al Qaeda and populated with foreign fighters, that have entered Syria to join the insurgency battling the forces of President Bashar al-Assad in the conflict, now in its third year.