The United States has lashed out at ally Israel over the “disgraceful” shelling of a United Nations school packed with refugees in Gaza.

In one of the most strongly-worded statements yet from Washington since the conflict began nearly a month ago, State Department spokeswoman Jen Psaki said on Sunday the United States was “appalled” by the attack on the school in Rafah.

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At least 10 people were killed in the shelling, the third time in 10 days a UN school has been hit in fighting amid growing international outrage over the civilian death toll in the nearly month-old conflict.

Thousands of Palestinians had sought refuge at the school from fierce fighting between Israeli forces and the Palestinian enclave’s Islamist rulers Hamas.

“The United States is appalled by today’s disgraceful shelling outside an UNRWA school in Rafah sheltering some 3000 displaced persons, in which at least 10 more Palestinian civilians were tragically killed,” Psaki said in a statement.

“We once again stress that Israel must do more to meet its own standards and avoid civilian casualties.”

She noted that the United Nations had “repeatedly” communicated to the Israeli Defense Forces the co-ordinates of the school and the world body’s other facilities.

Israel’s military confirmed it had fired on a target “in the vicinity” of the school.

“The IDF targeted three PIJ (Islamic Jihad) terrorists on board a motorcycle in the vicinity of an UNRWA school in Rafah,” an army statement said.

“The IDF is reviewing the consequences of this strike.”

Psaki’s statement said the presence of suspected militants near the facility could not excuse the attack, however.

“The suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians,” she said.

“We call for a full and prompt investigation of this incident as well as the recent shelling of other UNRWA schools.”

Valerie Jarrett, a senior adviser to President Barack Obama, said on CBS television’s “Face the Nation” program that while Israel was entitled to defend itself, civilian casualties were unacceptable.

“Israel absolutely has the right to defend itself and we are Israel’s staunchest ally,” Jarrett said.

“But you also can’t condone the killing of all of these innocent children. And so, we’re very concerned, we’re monitoring the situation closely,” Jarrett added, referring to an earlier attack on another school that left 16 people dead.

UN Secretary-General Ban Ki-moon decried the latest shelling as “a moral outrage and a criminal act.”

“This madness must stop,” Ban said through his spokesman, decrying what he termed “yet another gross violation of international humanitarian law.”