Blood of wounded and dead Palestinians is seen on the ground after an Israeli air strike outside a United Nations-run school in Rafah in the southern Gaza Strip.

The U.S. State Department Sunday issued its harshest public criticism of Israel during the current Gaza conflict — or in recent memory — calling the air strike outside of a United Nations-run school in Rafah "disgraceful" after 10 Palestinian civilians were killed.

Thousands of displaced Palestinians have used the school as a refuge.

"The suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians," spokeswoman Jen Psaki said in a statement emailed to reporters. "We call for a full and prompt investigation of this incident as well as the recent shelling of other UNRWA [U.N. Relief and Works Agency] schools."

The U.N. Secretary General Ban Ki-moon called the attack a "moral outrage and a criminal act" and a "gross violation of international law."

Israel declared a humanitarian cease-fire in most of Gaza for seven hours starting Monday at 10 a.m. local time. The cease-fire would apply to all of the Gaza Strip except eastern Rafah, Israeli officials said.