The United Nations expressed outrage on Wednesday over the killing of a Palestinian by an Israeli soldier who was caught on camera saying the evidence signalled a clear case of an extrajudicial execution.

Anger grew in the occupied West Bank on Tuesday after Israel's military court called the video evidence against the soldier "inconclusive", with activists demanding that Israel holds its army accountable for the killing of Abed al-Fattah Yusri al-Sharif, 21, earlier this month.

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A video of the incident showed the soldier step forward and shoot the Palestinian in the head at close range as he lay motionless on the ground. Fattah and another man, who was earlier shot and killed, were accused of stabbing an Israeli soldier in Hebron.

"There does not appear to be any provocation on the side of the gravely wounded man," said Christof Heyns, the UN's special rapporteur on extrajudicial, summary or arbitrary executions.

"The images shown carry all the signs of a clear case of an extrajudicial execution," Heyns said in a statement.

"Whatever legal regime one applies to the case, shooting someone who is no longer a threat is murder. It is furthermore troublesome that this was done to no apparent alarm to the other soldiers who were nearby."

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Also on Wednesday, Rupert Colville, spokesman for the UN High Commissioner for Human Rights, expressed his concerns "about the apparent extrajudicial execution" of Fattah.

Colville added there were worries the "killing may not be a lone incident.

"We urge the Israeli authorities to ensure all members of their security forces fully comply with their obligation to use force with restraint, only when strictly necessary, and in accordance with the principle of proportionality, and are constantly reminded that killing people who are no longer presenting an immediate threat is a crime, and will be treated as such," Colville said.

'It's a big lie'

The Israeli army arrested the soldier, but Lieutenant-Colonel Ronen Shor, the military court's judge, said on Tuesday there was "reasonable doubt" about the circumstances of the shooting "given the complexity of the events" surrounding a stabbing incident and the killing of the Palestinians that followed.

Shor also ruled that the soldier be held in custody for two more days. Prosecutors earlier said the shooting was "intentional" and asked the court to extend the soldier's detention until April 7 while the investigation is ongoing.

"It's a big lie, it was not the right of the soldier to shoot Fattah as the soldier was not in any sort of danger from any of the Palestinians," Issa Amro, director of the Hebron-based Youth Against Settlements activist group, told Al Jazeera.

"None of the Palestinians killed were found to have any explosives on them. The commander gave a direct order to the soldier to shoot and kill the Palestinian. This is a kind of revenge for them."

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Hundreds of Israeli protesters rallied outside the court on Tuesday in support of the soldier.

Demonstrators, as well as some politicians on the right, criticised Defence Minister Moshe Yaalon after he said the soldier acted inappropriately.

According to the poll conducted for Israel's Channel 2, about 57 percent of Israelis were against the soldier's arrest, and 42 percent of those questioned described the 19-year-old's actions as "responsible".

Only 5 percent classified the shooting as murder.

"The UN needs to establish an independent committee to investigate these extrajudicial killings, including that of Sharif's," said Amro.

"This was not the first case and it won't be the last case. There are videos out there for everyone to see.

"Also, it is the Israeli military system that needs to be held accountable, not just the soldier. These are war crimes and severe violations to international law. But I don't see anything happening in court."

Meanwhile, according to a Wednesday report by the US-based Politico magazine, Senator Patrick Leahy and 10 House members have sent a letter to John Kerry, the US Secretary of State, demanding investigations towards "gross violations of human rights" by Israel and Egypt's security forces.

Since October 1, Israeli forces or settlers have killed at least 206 Palestinians, including protesters, bystanders and attackers. Palestinian assailants have killed at least 29 Israelis, mostly in stabbing attacks.