WASHINGTON — The United States declared Sunday it is “appalled” by Israel’s “disgraceful” attack on a United Nations school sheltering some 3,000 displaced people in southern Gaza.

In language that was rare in its directness and severity, the U.S. denounced the attack earlier Sunday, which killed 10 people. The statement released by the U.S. State Department noted that the school had been designated a protected location.

“The coordinates of the school, like all U.N. facilities in Gaza, have been repeatedly communicated to the Israel Defense Forces,” State Department spokeswoman Jen Psaki said. “We once again stress that Israel must do more to meet its own standards and avoid civilian casualties.”

The State Department referred to the attack on the U.N. school as a “shelling,” although U.N. and Palestinian officials in Gaza called it an airstrike.

The U.S. condemnation follows one by U.N. Secretary-General Ban Ki-moon, who depicted the attack near the Rafah school as both “a moral outrage and a criminal act.”

Riyad Mansour, the Palestinian ambassador to the U.N., said putting the people of Gaza “in a continuous situation of confrontation and fighting” will only lead to more violence, adding, “This is an excellent atmosphere for radicalism.”

Pierre Krahenbuhl, head of the U.N. agency for Palestinian refugees, called the shelter attack a clear violation of international law. “These are premises that are protected, the sanctity of which has to be respected by all parties,” he said Sunday.