WASHINGTON - US Vice President Mike Pence has delayed a visit to the Middle East, the White House said, as a crunch tax vote on Capitol Hill looms and anger in the region over Washington's policy shift on Jerusalem persists.

The trip to Egypt and Israel, due to begin Tuesday, has been pushed back to mid-January, allowing Pence to remain in Washington in case he needs to cast the deciding vote in the Senate over President Donald Trump's tax reforms.

"The tax vote is still in very good shape, but we don't want to take any chances whatsoever," said a senior administration official.

Senator John McCain's return home to Arizona to fight cancer has left Republicans with a razor-thin margin to push the legislation over the finishing line.

Having failed to clear a series of legislative hurdles, the package is seen as key to Trump's ability to secure support among his base and skittish political donors.

"We have some senators who obviously can't make it there for the vote and the vice president feels that it's important for him to be here for the largest tax cut in history," the official said.

UN slams 'shocking' killing of disabled Palestinian protester The UN's human rights chief said Tuesday he was "truly shocked" by the Israeli army's killing of a wheelchair-bound Palestinian protester in Gaza, and demanded an "independent and impartial investigation".

Ibrahim Abu Thurayeh, a 29-year-old who lost both his legs in an Israeli attack a decade ago, was among five Palestinians killed on Friday during protests against US President Donald Trump's decision to recognise Jerusalem as the capital of Israel.

In a statement, UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein slammed Trump's decision as "dangerously provocative" and blamed it for the violence.

Following an initial investigation, the Israeli army said on Monday it had not deliberately targeted him and had found no evidence of any "moral or professional failures".

The UN office said Abu Thurayeh was among hundreds of people who marched across farmland towards the fence separating Gaza from Israel and appeared to have been shot in the head when he was 20 metres (yards) away from it.

"The facts gathered so far by my staff in Gaza strongly suggest that the force used against Ibrahim Abu Thurayeh was excessive," Zeid said.

"As far as we can see, there is nothing whatsoever to suggest that Ibrahim Abu Thurayeh was posing an imminent threat of death or serious injury when he was killed.

"Given his severe disability, which must have been clearly visible to those who shot him, his killing is incomprehensible - a truly shocking and wanton act."

In video footage recorded early on Friday, Abu Thurayeh could be seen holding the Palestinian flag and giving the victory sign to Israeli soldiers across the border.

Israel seized the largely-Arab eastern sector of Jerusalem during the 1967 Six-Day War and later annexed it, claiming both sides of the city as its "eternal and undivided capital."

But the Palestinians want the eastern sector as capital of their future state and have been infuriated by Trump's decision to recognise the city as the Israeli capital.

The UN rights office said Israeli forces had used live fire to disperse the protesters, indicating that the violence could be "traced directly back to the unilateral US announcement on the status of Jerusalem, which breaks international consensus and was dangerously provocative."

Zeid also called for "an independent and impartial investigation" to ensure perpetrators were held accountable.

On Monday, the Israeli army said its troops had been faced with a "riot" which was "extremely violent and included thousands of rioters."

In response, a "few controlled shootings were carried out towards main instigators," but "no live fire was aimed at Abu Thuraya," it said in a statement.

"It is impossible to determine whether Abu Thuraya was injured as a result of riot dispersal means or what caused his death," it said, adding: "The initial investigation indicates that no moral or professional failures were identified."