Mladenov called for an investigation into the death of a 15-year-old protester who was killed along the Gaza border.
Israel has said it will be taking disciplinary action over the video.
Seven tips for how Israel can improve its position amid turbulence on the Gaza border.
Joint List leader says video shows IDF soldiers as cold-blooded murderers, debunks Israel's claims of possessing the most moral military in the world.
The PA joined calls for international investigations into border incidents. The IDF has stated at that at least 10 of those killed were known terrorists.
"We will not allow a violation of Israeli sovereignty and we will use force if necessary."
William Schabas resigned as head of the UNHRC inquiry into Israeli war crimes.
The training was organized and held by members of Hamas’s armed wing, Izaddin al-Qassam.
The group said in a statement issued in the Gaza Strip that, “The enemy alone bears the responsibility of the repercussions of its uncalculated actions.”