Israel is due to be included on a UN "list of shame" of countries and armed forces failing to protect children in war.
The decision was welcomed by global rights groups Amnesty International and Human Rights Watch, with Amnesty Secretary-General Agnes Callamard saying that the decision was long overdue.
"It should not have taken 15,000 children killed in Gaza for Israel to be on this shameful list," she tweeted.
"We can't confirm it, but if it's true, it's a thoroughly justified, albeit long overdue, move by the secretary-general," said Louis Charbonneau from Human Rights Watch.
However, the decision prompted a furious Israeli response on Friday.
"I am utterly shocked and disgusted by this shameful decision," Israeli envoy to the UN Gilad Erdan said in a statement.
Israeli Prime Minister Benjamin Netanyahu posted on his X social media account that the UN had "added itself to the blacklist of history when it joined those who support the Hamas murderers."
The annual "Children and Armed Conflict" report from UN Secretary-General Antonio Guterres is not due to be published until June 18, but Erdan went public after receiving notification of Israel's inclusion on the list of countries that do not take adequate measures to shield children from conflict.
A diplomatic source told AFP that Hamas and another Palestinian militant group, Islamic Jihad, would also appear on the list.
Gaza has been utterly devastated by eight months of indiscriminate Israeli war. At least 36,731 people in Gaza, including 15,500 children, have been killed according to local authorities.
Israel has also delayed the entry of aid into Gaza, depriving the territory's 2.4 million people of clean water, food, medicines and fuel.
Last week, the World Health Organization said that more than four in five children had gone a whole day without eating at least once in 72 hours.
At least 32 people, many of them children, have died of malnutrition in Gaza since the war began.
The UN report highlights human rights violations against children in around 20 conflict zones. Last year, Russia's military and armed entities linked to Russia were included on the list.
Rights groups have long pushed for Israel's inclusion and in 2022, the United Nations issued a warning that Israel would need to show improvements in order not to be added.