"There are no words to fully express our condemnation of Israel's bombing of a #Gaza hospital today, killing hundreds of people," Faki said on X, formerly Twitter, calling for the international community to act.
The Palestinian health ministry said Israeli air strikes on the hospital compound sheltering displaced people had killed at least 600 people.
"Targeting a hospital, considered a safe haven under International Humanitarian Law, is a war crime," Faki said.
Israel has been bombing targets in Gaza since Hamas militants from the territory attacked southern Israel on 7 October.
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.
* This story was edited by Ahram Online
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