Theresa May has backed calls for “an independent investigation” into Israel’s killing of 64 Palestinian protesters, including children, and injuring more than 2,000 others at the Gaza border this week.

Labour leader Jeremy Corbyn described it as an “outrage” and a “wanton disregard for international law” while Shadow Foreign Secretary Emily Thornberry said the “vicious and utterly avoidable slaughter” needed an urgent investigation.

Middle East Minister Alistair Burt said the “large volume of live fire is extremely concerning” and May later added that she was “deeply troubled” by the killings, which came as American dignitaries flew in to open the new Jerusalem embassy.

Up to 40,000 Gaza residents took part in Monday’s ‘Great March of Return’. While mostly peaceful, some threw stones and Molotov cocktails at Israeli soldiers stationed on the border, while others flew kites soaked with petrol.

In response, the IDF used drones to drop tear gas on protesters, while troops fired rubber bullets and live ammunition, describing its actions as self-defence.

“There is an urgent need to establish the facts of what happened,” said May during a joint press conference with Turkish President Recep Tayyip Erdogan, who expelled the Israeli ambassador from Ankara in protest.

May said the events warranted an “independent investigation” – typically such incidents are investigated by the Israeli military – to answer questions such as “why such a volume of live fire was used and what role Hamas played in events”.

She added that the loss of life was “tragic and extremely concerning… Such violence is destructive to peace efforts and we call on all sides to show restraint”.

In Parliament, Thornberry said the IDF used ammunition designed for hunters to “do maximum internal damage to the animal” but IDF spokesman Col. Jonathan Conricus defended soldiers’ conduct.

“We resort to live fire only when absolutely necessary and when there is clear threat to infrastructure or to Israeli soldiers,” he said. “If there is, then we use snipers who fire specifically and under very clear guidance by commanders.”

In an interview with an American news network, Israeli Prime Minister Benjamin Netanyahu said Hamas was “pushing civilians – women, children – into the line of fire with a view of getting casualties,” adding: “We try to minimise casualties. They try to incur casualties in order to put pressure on Israel, which is horrible.”

But Corbyn said the killings were a “flagrant illegality” and that the UK’s response had been “wholly inadequate,” adding: “We cannot turn a blind eye to such wanton disregard for international law. That is why Labour is committed to reviewing UK arms sales to Israel while these violations continue.”

UN High Commissioner for Human Rights Zeid Ra’ad al-Hussein said “those responsible for outrageous human rights violations must be held to account” while Kuwait called for an urgent meeting of the UN Security Council, only to be initially vetoed by the United States, whose ambassador Nikki Haley defended Israel.

“The Hamas terrorist organisation has been inciting violence for years, long before the United States decided to move our embassy,” she said. “No country in this chamber would act with more restraint than Israel has.”

Kuwait’s U.N. Ambassador Mansour al-Otaibi said that, as an occupying power, Israel was required under the Geneva Convention to protect Palestinian civilians “but they failed to do that… this is why we want the council to do something”.

But Netanyahu said: “I don’t know of any army that would do anything differently if you had to protect your border against people who say ‘We’re going to destroy you, we’re going to flood into your country.’”

He added: “If Hamas had not pushed them there, then nothing would happen. Hamas holds responsibility for doing this, and they’re deliberately doing it.”

As tens of thousands attended funerals in Gaza on Tuesday, Hamas’s political chief Ismail Haniyeh described Monday’s violence as a “massacre,” in what was the deadliest day in the Israeli-Palestinian conflict since war in the Strip in 2014.

Gaza residents have been protesting at the border every week since the end of March. The death-toll of these demonstrations has now passed 100, and Monday’s clashes has brought renewed focus on Israel’s rules of engagement.

Last month, six Israeli human rights groups lodged an urgent petition with Israel’s High Court, saying: “The rules of engagement concerning Gaza permit live fire at protesters classified by the IDF as ‘key agitators’ or ‘major disturbers of the peace’ even when they do not pose a clear and immediate threat to human life”.

The group, led by Yesh Din, added that “the orders also permit soldiers to shoot at demonstrators for merely approaching the Gaza-Israel fence”.

Looking ahead to the coming weeks and months, Professor Menachem Klein of Bar-Ilan University and Visiting Professor at King’s College, London, said the next flare-up was a question of when, not if.

“Every Friday during Ramadan, tens of thousands, maybe even hundreds of thousands of Palestinians come to the Al-Aqsa Mosque [in Jerusalem] to pray, and Jerusalem is the centre, so this will be very tense,” said Klein.

“Also, big protests are expected on 5 June, on the anniversary of the start of the Six Day War, and this falls in the holy month of Ramadan, which begins this week. So don’t think it will disappear. It may calm down, but only temporarily. It is only a matter of time.”