REUTERS - A UN expert on human rights on Wednesday condemned the killing by an Israeli soldier of a wounded Palestinian assailant last week as he lay on the ground, saying it appeared to be an extrajudicial execution.

Video taken by Israel's B'Tselem human rights group last Thursday shows an infantryman firing into the head of a Palestinian as he lay on the ground in Hebron in the occupied West Bank. Minutes earlier, the Palestinian had stabbed and wounded another soldier.

The soldier has been arrested, though not yet charged, by Israeli authorities. The military initially said it was a murder investigation, but on Tuesday prosecutors told a court they were looking into possible manslaughter charges.

The incident brought to a boil a debate in Israel over whether excessive force has been used against Palestinian assailants.

"The images shown carry all the signs of a clear case of an extrajudicial execution," said Christof Heyns, UN special rapporteur on extrajudicial, summary or arbitrary executions, of the Hebron shooting. "Whatever legal regime one applies to the case, shooting someone who is no longer a threat is murder."

On Thursday, an autopsy will be carried out on the Palestinian's body, with a Palestinian doctor present, and the findings could be key in determining the severity of charges to be brought against the soldier.

There is still a gag order regarding the soldier's identity.

Israel's military chief on Wednesday sent out a letter to troops in which he said the army will support any soldier that makes a mistake in the heat of the battle, but at the same time it will hold accountable soldiers or commanders who do not adhere to the military's moral code.

The past six months has seen the worst period of sustained violence in the West Bank, Jerusalem and the Israeli interior since a Palestinian uprising ended a decade ago.

A campaign of Palestinian knife, car-ramming or gun assaults have killed 28 Israelis and two U.S. citizens. At least 190 Palestinians, 129 of whom Israel says were assailants, have been killed by security forces and many others were shot during clashes and protests.