BEIRUT, Lebanon — The top United Nations human rights official warned opposition fighters in Syria on Monday that they would not be immune from prosecution for atrocities, as videos from the Syrian city of Aleppo appeared to show a mass execution by rebel fighters of bound and blindfolded Syrian government soldiers.

One of the videos, first publicized on Monday on the Brown Moses blog, which curates and analyzes video evidence from Syria, showed at least 20 corpses lying in a crooked row on a bloodstained street curb. The victims wore fatigues but no shoes. Several appeared to have been shot in the head.

In that video and another that captured the same scene, different rebel groups appear to take responsibility for the killings. It was impossible to immediately confirm the authenticity of the videos, or to determine exactly when and where they were recorded. If confirmed, the executions were likely to add to growing concerns about the conduct of the militias fighting to topple the Syrian president, Bashar al-Assad, and particularly their treatment of prisoners.

In a brutal episode in late July, a group of rebel fighters was seen in a video executing several captives — members of an Aleppo family accused of being enforcers for the government — with a spray of gunfire. In recent days, other videos have captured summary executions by the rebels.