Israel's prime minister is defending the military after footage emerged of a soldier lethally shooting a Palestinian who had already been shot and subdued. Benjamin Netanyahu has previously said the incident in question does not reflect the military's conduct, and on Sunday said criticism of the armed forces as a whole over the incident is "outrageous and unacceptable." The military has arrested the soldier and opened an investigation into what it said appeared to be a "grave breach" of its values. Several right-wing lawmakers have come to the soldier's defense, accusing detractors of abandoning him before he was given a fair hearing. Hundreds protested outside the prison in which he is being held, and posters have surfaced denouncing the defense minister and military chief. The United Nations condemned on Friday the "gruesome" killing of the wounded Palestinian after a video of the shooting spread widely online. "I strongly condemn yesterday's apparent extra-judicial execution of a Palestinian in Hebron in the occupied West Bank," UN Special Coordinator for the Middle East peace process Nickolay Mladenov said in a statement. "This was a gruesome, immoral and unjust act that can only fuel more violence and escalate an already volatile situation." He welcomed, however, Israeli Defense Minister Moshe Yaalon's condemnation of the incident and called on the Israeli authorities to "swiftly bring to justice" the alleged perpetrator. The Israeli army on Thursday arrested a soldier caught on video shooting a wounded Palestinian assailant in the head as he lay on the ground after stabbing another soldier, his knife lying beyond his reach. The clip, which was widely shared online and shown by state-owned and commercial Israeli TV channels, shows what appears to be one of the most flagrant cases of Israeli forces' alleged use of excessive force so far in the wave of Israeli-Palestinian violence that erupted in October. The incident was immediately condemned as an "execution" by Israeli rights group B'Tselem and as a "war crime" by Palestinians. A military spokesman said the soldier was being "detained on suspicion of murder," with his remand extended to Tuesday. The soldier had not been at the actual site of stabbing attack, but arrived some six minutes later, according to a preliminary after action review. Three officers on the scene were also reprimanded for not treating the wounded Palestinian, a military spokesman said, noting that in the past six months some 170 Palestinians wounded during attacks or attempted attacks received medical treatment from army forces. Military chief of staff Lieutenant General Gadi Eisenkot said Friday that the army gives its soldiers "full support in the case of errors, but not over conduct that is not normative and goes against the army's values."