Israel hit a Gaza school on Thursday with what it described as a targeted airstrike on up to 30 Hamas and Islamic Jihad fighters inside.
A Hamas official said 40 people were killed, including women and children sheltering at the U.N. site.
Video footage showed Palestinians hauling away bodies and scores of injured in a hospital after the attack, which took place at a sensitive moment in mediated talks on a cease-fire that would involve releasing hostages held by Hamas and some of the Palestinians held in Israeli jails.