UN Secretary General Ban Ki-moon condemned an Israeli strike on a Gaza school that killed 16 people as "unjustifiable" Wednesday, calling for those responsible to be held to account.
"This morning a UN school sheltering thousands of Palestinian families suffered a reprehensible attack," Ban said during a visit to Costa Rica.
"It is unjustifiable, and demands accountability and justice."
The school in the Jabaliya refugee camp in northern Gaza, which was sheltering more than 3,000 displaced people, came under attack from Israeli tank shells at dawn Wednesday.
It is the second time in a week that a UN school housing refugees has been hit.
Ban accused the Israeli military of ignoring repeated communications on the location of the school.
"I want to make it clear that the exact location of this elementary school has been communicated to the Israeli military authorities 17 times, as recently as last night, just a few hours before this attack," he said.
"They are aware of the coordinates and exact location of where these people are."
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Costa Rican President Luis Guillermo Solis meanwhile joined international calls for an end to the bloodshed.
"We want the immediate establishment of a UN-monitored ceasefire to enable an escape from this bloody vortex, which is an offense to humanity," he said.
Speaking at UN headquarters in New York, Deputy Secretary General Jan Eliasson voiced his "shock" and "dismay" at the attack.
"They were there under UN protection, under our protection," he said of the refugees.
He stressed that civilians seeking to flee the fighting had "nowhere to go" given that Gaza's borders with neighboring countries were shut.
"This is a moment when you really have to say 'enough is enough,'" he said.
John Ging, the director of operations for the UN Office for the Coordination of Humanitarian Affairs, said 229 children have died in the latest fighting between Israel and Hamas, 21 percent of all casualties.
He said a "child per hour is killed" in Gaza.
More than 1,300 people have been killed in 23 days of unrelenting attacks by Israel, which it says is aimed at halting Palestinian rocket fire.