Israel has demanded that the UN's secretary general retract comments he made about the Gaza war and apologise.
António Guterres said in a speech to the Security Council on Tuesday that he condemned unequivocally Hamas's deadly attacks in Israel two weeks ago but that they "did not happen in a vacuum".
Israeli ambassador Gilad Erdan accused him of "justifying terrorism" and called for his immediate resignation.
On Wednesday, Mr Guterres rejected "misrepresentations" of his statement.
But Mr Erdan said in reply that the UN chief "once again distorts and twists reality" and called for his resignation.
On 7 October, some 1,500 Hamas gunmen infiltrated southern Israel from Gaza. They killed at least 1,400 people, most of them civilians, and took another 222 people as hostages.
The Hamas-run health ministry in Gaza says more than 6,500 people have been killed in the territory since Israel retaliated with air and artillery strikes while massing troops for an expected ground invasion.
Addressing a meeting of the UN Security Council in New York on Tuesday, Mr Guterres urged all parties in the war to respect and protect civilians.
"I have condemned unequivocally the horrifying and unprecedented 7 October acts of terror by Hamas in Israel. Nothing can justify the deliberate killing, injuring and kidnapping of civilians - or the launching of rockets against civilian targets."
He then told the council that it was "important to also recognize the attacks by Hamas did not happen in a vacuum", adding: "The Palestinian people have been subjected to 56 years of suffocating occupation."
He described how Palestinians had "seen their land steadily devoured by settlements and plagued by violence; their economy stifled; their people displaced, and their homes demolished".
"But the grievances of the Palestinian people cannot justify the appalling attacks by Hamas. And those appalling attacks cannot justify the collective punishment of the Palestinian people."
Mr Guterres also said he was deeply concerned about "the clear violations of international humanitarian law that we are witnessing in Gaza".
He expressed alarm at Israel's continuous bombardment of Gaza, as well as the level of civilian casualties and "wholesale destruction of neighbourhoods".
Without naming Hamas, he stressed that "protecting civilians can never mean using them as human shields".
And without naming Israel, he said: "Protecting civilians does not mean ordering more than one million people to evacuate to the south, where there is no shelter, no food, no water, no medicine and no fuel, and then continuing to bomb the south itself."
The UN chief also appealed for a humanitarian ceasefire to make the delivery of aid to Gaza easier and safer, and to facilitate the release of the hostages.
He called the crossing from Egypt of 62 lorries carrying food, water and medical supplies since Saturday "a drop of aid in an ocean of need".
He warned that the failure to include fuel risked a disaster, explaining that hospitals would be left without power and drinking water would not be purified or pumped.
The foreign minister of the West Bank-based Palestinian Authority, Riyad al-Maliki, demanded an end to what he called the "ongoing massacres being deliberately and systematically and savagely perpetrated by Israel" against the two million people living in Gaza.
Visiting Israeli Foreign Minister Eli Cohen criticised Mr Guterres in his speech to the Security Council, asking him: "In what world do you live?"
Mr Cohen said the killing of 1,400 men, women and children by Hamas constituted a massacre that would "go down in history as more brutal" than those committed by the Islamic State (IS) group.
"Hamas are the new Nazis," Mr Cohen declared. "Just as the civilised world united to defeat the Nazis, just as the civilised world united to defeat [IS], the civilised world has to stand united behind Israel to defeat Hamas."
Addressing the UN's appeals for proportionality and a ceasefire, he said: "Tell me, what is a proportionate response for killing of babies, for rape [of] women and burn them, for beheading a child? How can you agree to a ceasefire with someone who swore to kill and destroy your own existence?"
Mr Cohen later wrote on X, formerly known as Twitter: "I will not meet with the UN secretary general. After the October 7th massacre, there is no place for a balanced approach."
Mr Erdan said the secretary general had "expressed an understanding for terrorism and murder".
On Wednesday, the ambassador was quoted by Israeli news website Ynet as saying he had informed the UN's Under-Secretary General for Humanitarian Affairs, Martin Griffiths, that his request for an Israeli visa had been refused because of Mr Guterres's remarks.
"He will not be able to come here to the region. Their agencies constantly need to bring in new people, certainly at a time like now. They will be refused."
A spokesman for UK Prime Minister Rishi Sunak said "there is and can be no justification for Hamas's barbaric terrorist attack which was driven by hatred and ideology".
Later, Mr Guterres told reporters: "I am shocked by the misrepresentations by some of my statement... as if, as if I was justifying acts of terror by Hamas. This is false. It was the opposite."
"I believe it was necessary to set the record straight, especially out of respect to the victims and to their families," he added.
But Mr Erdan said it was a "disgrace to the UN that the secretary general does not retract his words and is not even able to apologise for what he said yesterday".
"Every person understands very well that the meaning of his words is that Israel has guilt for the actions of Hamas or, at the very least, it shows his understanding for the 'background' leading up to the massacre," he said.
"A secretary general who does not understand that the murder of innocents can never be understood by any 'background' cannot be secretary general... I again call on him to resign."
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