Israeli firing into Hamas-ruled Gaza killed nearly 60 Palestinians at mass border protests on Monday.
“There is little evidence of any attempt to minimise casualties on Monday,” Zeid Ra’ad al-Hussein told a special session of the UN Human Rights Council in Geneva.
The council voted through the resolution with 29 in favour and two opposed, while 14 states abstained.
The resolution also condemned “the disproportionate and indiscriminate use of force by the Israeli occupying forces against Palestinian civilians”.
Israel condemned the resolution, which was put forward by a group of countries including Pakistan. The United States decried it as an example of a biased focus on Israel by the council.
Both lamented that it didn’t mention Gaza’s Hamas rulers, whom Israel blames for the violence.
The “independent, international commission of inquiry” mandated by the council will be asked to produce a final report next March.
Commenting on the UK’s abstention from the vote, the London-based anti-poverty charity War on Want told The Independent: “It’s disgraceful that the UK has abstained from a UN Human Rights Council vote for a Commission of Inquiry into Israel’s killing of unarmed Palestinians in Gaza.”
Ryvka Barnard, the charity’s senior campaigner on militarism and security, said: “In doing so, the UK has gone against a broad international consensus amongst leading international, Palestinian and Israeli human rights organisations, which agree that the only way to approach justice is through an independent inquiry.
“Trusting that Israel can fairly and neutrally investigate its own war crimes is an insult to any common sense definition of justice, and makes a mockery of the UK’s stated commitment to international law, human rights, and accountability.”
In a vigorous speech, Mr Zeid slammed the “appalling” recent events in Gaza and called for the occupation of Palestine by Israel to end.
He said the 1.9 million people living in Gaza had been denied human rights by Israeli authorities and described those living in the Palestinian enclave as “caged in a toxic slum from birth to death”.
“They are, in essence, caged in a toxic slum from birth to death; deprived of dignity; dehumanised by the Israeli authorities to such a point it appears officials do not even consider that these men and women have a right, as well as every reason, to protest,” he said.
The vote for an investigation came days after Israeli forces shot and killed 59 Palestinians and injured more than 2,700 during mass protests along the Gaza border on the day the US officially opened its embassy in Jerusalem.
Mr Zeid said that under international law, Israel was obligated to protect the population of Gaza and ensure their welfare, “but there is little evidence of any attempt to minimise casualties,” he added.
The human rights chief said 118 Palestinians, including 15 children, were killed since protests began on 30 March. He said the number continues to climb as some of the injured die from their wounds.
He compared the Palestinians’ use of Molotov cocktails, slingshots and burning kites against the “horrifying and criminal violence” with which they were met.
“The stark contrast in casualties on both sides is ... suggestive of a wholly disproportionate response. Killings resulting from the unlawful use of force by an occupying power may also constitute ‘wilful killings’ – a grave breach of the Fourth Geneva Convention,” he told the UN council.
“Nobody has been made safer by the horrific events of the past week,” he concluded.
Backing calls for an independent investigation into the events “in the hope the truth regarding these matters will lead to justice”, Mr Zeid warned “those responsible for violations must in the end be held accountable”.
“What do you become when you shoot to kill someone who is unarmed, and not an immediate threat to you? You are neither brave, nor a hero. You have become someone very different to that,” he said.
Turkey’s president, Recep Tayyip Erdogan, said he will take the events in Gaza to the United Nations General Assembly.
Speaking at the start of an extraordinary summit of Muslim nations held in Istanbul in response to the week’s events, Mr Erdogan said the United States blocked every step towards justice in the UN Security Council.
“Israel must certainly be held accountable for the innocent people it has massacred in front of international law. We will follow this issue in the United Nations General Assembly as well,” he said.
Earlier this week, the UK called for a UN investigation to look at why “such a volume” of live ammunition was used by Israeli troops against Palestinians protesters in Gaza.
Foreign minister Alistair Burt said UK diplomats were “urgently” working towards trying to establish an inquiry after the violence which followed Donald Trump’s move to formally recognise Jerusalem as Israel’s capital rather than Tel Aviv.
The human rights commissioner’s speech comes after Egypt’s president, Abdel-Fattah el-Sisi, ordered the opening of the Rafah border crossing with Gaza for the entire Muslim holy month of Ramadan – the longest length of time since 2013.
In a tweet, Mr Sisi said the opening of the border would “alleviate the burdens of the brothers in the Gaza Strip”.
The crossing has been open since Saturday so Mr Sisi’s announcement is technically an extension.
The high number of wounded following this week’s violence has overwhelmed the Gaza health system: Egyptian authorities said 510 people crossed the border on Wednesday alone, the majority coming from Gaza.
Officials said dozens of trucks with medical aid and food from the Red Crescent and cement, steel and power engines had travelled from Egypt into Gaza.
The Rafah crossing is Gaza’s main gate to the outside world but has only had sporadic openings since the 2013 ouster of Egypt’s elected Islamist president, Mohammed Morsi, a high-ranking member of Hamas’ parent group the Muslim Brotherhood.
In 2007, Hamas wrested control of Gaza by force, provoking the Israeli-Egyptian blockade that severely restricted the movement of most of Gaza’s two million inhabitants.
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