Those responsible for the deaths and injuries of Palestinian children this month must be brought to account, the UN rights chief said on Thursday.
The call follows the three days of intense conflict between Israel and Islamic Jihad militants in the densely populated Palestinian enclave of Gaza last week.
"Inflicting hurt on any child during the course of conflict is deeply disturbing," said Michelle Bachelet, the UN High Commissioner for Human Rights.
"The killing and maiming of so many children this year is unconscionable."
Israel said its air and artillery strikes were aimed at positions of the Iran-backed Islamic Jihad group.
Ms Bachelet's office said 19 Palestinian children had been killed in the recent unrest, taking the total number this year to 37.
The latest child killed was Liyan Al Shaer, 10, who died in a Jerusalem hospital on Thursday from a head wound sustained in the latest round of Gaza violence.
Seventeen children were killed during the Gaza hostilities from August 5 to 7, while two more were killed on Tuesday in Israeli law enforcement operations in the West Bank.
Gaza Strip hit by Israeli strikes — in pictures
The UN Human Rights Office said that among the 48 Palestinians killed in last week's Gaza conflict, there were at least 22 civilians. They included the 17 children.
Of the 360 Palestinians reported injured, nearly two thirds were civilians, including 151 children, the high commission said.
Three Gazan children are on life-support systems at a hospital in Israeli-annexed East Jerusalem, the official Palestinian news agency Wafa reported Thursday.
"In a number of incidents, children were the majority of casualties," Ms Bachelet's office said.
"Launching an attack which may be expected to incidentally kill or injure civilians, or damage civilian objects, in disproportionate manner to the concrete and direct military advantage anticipated, is prohibited.
"Such attacks must stop."
Residents in Gaza Strip welcome ceasefire between Israel and militants - video
Residents in Gaza Strip welcome ceasefire between Israel and militants
Ms Bachelet's office said that, also in violation of international humanitarian law, Palestinian armed groups launched hundreds of rockets and mortars in indiscriminate attacks, causing civilian casualties in both Israel and Gaza.
Israel insists that some of the civilian deaths including children were killed by Islamic Jihad rockets that fell short or misfired.
Ms Bachelet called for investigations into all incidents where any person was killed or injured.
"An almost total lack of accountability persists in the occupied Palestinian territory," she said.
"Whether for violations of international humanitarian law by all parties in hostilities in Gaza, or for recurring Israeli violations of international human rights law and the law of occupation in the West Bank, including East Jerusalem."
