An Israeli soldier was caught on video Thursday shooting a wounded Palestinian assailant in the head as he lay on the ground, with footage of the incident spreading widely online.

The video threatened to further inflame tensions amid a wave of Israeli-Palestinian violence that erupted in October.

It showed the Palestinian, who along with another man had allegedly stabbed a soldier in the West Bank city of Hebron, lying on the ground, apparently after being shot.

The soldier then shoots him again, in the head, without provocation. The Palestinian, identified as 21-year-old Abdul Fatah al-Sharif, was killed.

The soldier was detained, with Defence Minister Moshe Yaalon vowing that the incident would be treated with "utmost severity".

"The event in which a soldier is recorded arriving at the scene of the attack in Hebron and shooting the terrorist minutes after he was already neutralised and is lying on the ground is most grave and is in clear contradiction of the (military's) values and battle ethics," he said in a statement.

Military spokesman Peter Lerner said "all of the soldiers involved will be investigated, as will the orders given."

The army said the investigation began before the video surfaced online.

Palestinian health minister Jawad Awwad called it a "war crime," saying that even prior to the shooting medics on site were not treating the wounded man.

Violence since October 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 Israeli forces have also been accused of using excessive force in some cases, chargers which they have firmly denied.

In the video, the man -- who appears to be seriously injured -- is lying on the ground while Israeli medics treat the injured soldier.

It then shows an Israeli soldier nearby raising his rifle and shooting the Palestinian in the head.

- 'Potential war crime' -

The video sparked condemnation.

Sarit Michaeli, a spokeswoman for Israeli rights group B'Tselem, which posted the video, called it an "execution".

"It is clear that the Palestinian youth who was one of the two who stabbed the soldier was lying on the ground without posing any risk to security services," she told AFP.

"The security personnel around him are not treating him in any way like he is a danger. The soldier is simply seen shooting him in the head."

Amnesty International said the shooting had "absolutely no justification and must be prosecuted as a potential war crime."

The incident came after an alleged stabbing minutes earlier as Jews marked the holiday of Purim, which sees street parades with costumed revellers.

The already heavy Israeli security presence in the flashpoint city of Hebron has been increased for the holiday.

According to the army, the two knife-wielding Palestinians wounded an Israeli soldier at the entrance to the heavily guarded Jewish settler enclave in the heart of the city.

Settlements within Hebron are a source of constant tension with its 200,000-strong Palestinian population.

In an initial statement on the incident, the Israeli army said only that the two assailants had been shot dead. The second Palestinian was identified as Ramzi al-Qasrawi, also 21.

Medics said the soldier wounded in the stabbing was in moderate condition.

Beyond the increased security within Hebron, Israel also took the unusual decision of closing off the West Bank for Purim, with the security measure that affects tens of thousands of Palestinian workers to remain in place through Saturday.

It regularly implements such closures for other Jewish holidays such as Passover and Yom Kippur, though less often for Purim.

- Religious duty -

Sharif's killing is likely to spark fresh allegations that Israel has used disproportionate force as it seeks to quell more than five months' of violence.

Military chief of staff Gadi Eisenkot made waves when he warned against excessive force during a speech in February.

Eisenkot said that "when there's a 13-year-old girl holding scissors or a knife and there is some distance between her and the soldiers, I don't want to see a soldier open fire and empty his magazine at a girl like that."

In an apparent response, an Israeli chief rabbi said it was a religious duty to kill attackers.