Jerusalem (AFP) - The United Nations condemned on Friday the "gruesome" killing of a wounded Palestinian assailant by an Israeli soldier in the West Bank, after a video of the shooting spread widely online.

"I strongly condemn yesterday's apparent extra-judicial execution of a Palestinian assailant 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 Defence 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."

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Israeli Prime Minister Benjamin Netanyahu said the soldier's behaviour was not in keeping with the army's values and Yaalon vowed the shooting was being treated with "utmost severity".

The killing also fired up Israeli politicians and added heat to the debate over Israeli forces' response to the current wave of violence which has left 200 Palestinians and 28 Israelis dead.

Most of the Palestinians killed were carrying out knife, gun or car-ramming attacks, according to Israeli authorities.

But the Israelis have also been accused of using excessive force in some cases, charges which they have firmly denied.

Education Minister Naftali Bennett said criticism of the soldier involved in Thursday's killing was damaging to the country.

"The attacks on a soldier who protects all of us before the investigation has even started harms the military, harms Israel's standing in the world, and is harmful towards our attempts to thwart future attacks," he said.