AMMAN -- The United States came out with its harshest criticism to date of Israel's military operation in Gaza, calling the shelling of a United Nations Relief and Works Agency facility in Gaza on Sunday "disgraceful."

"The United States is appalled," State Department spokeswoman Jen Psaki said in a statement on Sunday. "The coordinates of the school, like all UN facilities in Gaza, have been repeatedly communicated to the Israeli Defense Forces."

Ten civilians, including children, reportedly died from the attack, which hit just outside a school in Rafah sheltering over 3,000 displaced Palestinians.

"Israel must do more to meet its own standards and avoid civilian casualties," Psaki continued. "The suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians."

The Israeli government has initiated an investigation into the incident. Sunday's shelling marks the third such strike in as many weeks on a school, and in at least one past incident, the Israel Defense Forces said that militants had been using the facility as a launching pad for attacks.

Psaki said the US would seek a "full and prompt investigation" into the event, and underscored the need for "all parties" to "comply with international humanitarian law."

Earlier Sunday, UN Secretary General Ban Ki-moon called the attack "a moral outrage and a criminal act."

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