WASHINGTON - The U.S. government issued its strongest criticism of Israel to date today, after shelling near a UN school sheltering civilians took at least 10 Palestinian lives in Gaza.

"The United States is appalled by today's disgraceful shelling outside an UNRWA school in Rafah sheltering some 3,000 displaced persons," State Department spokeswoman Jen Psaki said.

"The suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians," her written statement read.

The statement also included an admonishment to Hamas that "UN facilities, especially those sheltering civilians, must be protected, and must not be used as bases from which to launch attacks."

Psaki's statement marked the first time during the current Gaza conflict that the U.S. has said explicitly that suspected Hamas activity in the area does not justify Israeli attacks that risk civilian lives.

Calling for a "full and prompt investigation," Psaki referenced international law and said that "Israel must do more to meet its own standards and avoid civilian casualties."

It was the second time in as many weeks that a UN facility saw close shelling after the UN said it gave Israel specific coordinates of where civilians were being sheltered and asked Israel not to attack.

As with the first episode, this one sparked outrage from the UN.

"The attack is yet another gross violation of international humanitarian law, which clearly requires protection by both parties of Palestinian civilians, UN staff and UN premises, among other civilian facilities," the UN said in an official statement from Secretary-General Ban Ki-Moon's spokesperson.

Throughout the conflict, President Obama, Secretary of State John Kerry, and State Department officials have called on Israel to take additional steps to protect civilian lives, expressing "serious concern" while recognizing Israel's "right to defend itself" as long as Hamas fires rockets toward Israel. Today's statement marked a significant escalation of how far the U.S. is willing to go in criticizing its ally.

Israel and its supporters have maintained throughout the conflict that Hamas bears responsibility for launching attacks from civilian neighborhoods and storing weapons among the civilian population.

Soon after Israel invaded, schools in Gaza run by the United Nations Relief and Works Agency for Palestine Refugees (UNRWA) discovered rockets stored on their premises.