Around 3,000 have been killed in Israeli air strikes across the Gaza since war began after Hamas attack.
The deadly strike on a hospital in the Gaza Strip on Tuesday is "totally unacceptable", the UN human rights chief said, insisting that the perpetrators must be held to account.
"Words fail me," Volker Turk said in a statement.
"Hundreds of people were killed -- horrifically -- in a massive strike at Al Ahli Arab Hospital in Gaza City, including patients, healthcare workers and families that had been seeking refuge in and around the hospital.
"Once again the most vulnerable. This is totally unacceptable."
The health ministry in the Hamas-run Palestinian territory said Israeli air strikes on the hospital compound sheltering displaced people had killed at least 200 people.
The Israeli army said the strike was a rocket misfired by the Gaza-based militant group Islamic Jihad.
"Hospitals are sacrosanct, and they must be protected at all cost," said Turk.
"We don't yet know the full scale of this carnage, but what is clear is that the violence and killings must stop at once."
Besides the hospital strike, Turk noted that at least six people were killed earlier Tuesday when a UN-run Gaza school they were sheltering in was hit. At least 4,000 people had taken refuge there.
"All states with influence must do everything in their power to bring an end to this horrendous situation," the UN high commissioner for human rights said.
"Civilians must be protected, and humanitarian aid must be allowed to reach those in need as a matter of urgency.
"Those found responsible must be held to account."
Israel has been bombing targets in Gaza since Hamas militants from the territory attacked southern Israel on October 7, triggering a war which has left thousands of dead on both sides.
Around 3,000 people have since been killed in Israeli air strikes across the Gaza Strip.
More than 1,400 people in Israel have been killed, the majority gunned down by Hamas militants who crossed from Gaza and attacked border communities.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)