NEW YORK, N.Y. – U.N. Secretary-General Ban Ki-moon condemned an attack that killed 10 people at a U.N. school in the Gaza Strip on Sunday as “a moral outrage and a criminal act.”

In his most strongly worded statement yet on an attack against a U.N. facility in Gaza, Ban called it “yet another gross violation of international humanitarian law, which clearly requires protection by both parties of Palestinian civilians, U.N. staff and U.N. premises, among other civilian facilities.”

“This attack, along with other breaches of international law, must be swiftly investigated and those responsible held accountable. It is a moral outrage and a criminal act,” Ban said.

A Gaza health official said at least 10 people were killed and 35 wounded after the strike near the boys’ school in Rafah. Several bodies, among them children, were strewn inside the U.N. school’s compound. At least one U.N. staffer was killed, said Robert Turner, the director of operations for the U.N. Palestinian refugee agency in Gaza.

Turner said preliminary findings indicated the blast was an Israeli airstrike near the school.

At least six U.N. facilities have been struck by Israeli fire since the conflict between Israel and Hamas began. After an attack that killed at least 16 people at a U.N. school last week, Ban said “nothing is more shameful than attacking sleeping children.”

In Sunday’s statement, Ban reiterated that Israel’s military has “been repeatedly informed of the location of these sites.”

The Israeli military had no immediate comment on the strike, which came as Israel signalled a possible scaling back in the ongoing war.

The U.N. chief also expressed his frustration with the unraveling of the 72-hour ceasefire last weekend, which he had helped broker along with U.S. Secretary of State John Kerry. Ban earlier blamed Hamas for violating the cease-fire.

Ban spent six days in the Middle East last month and countless hours on the phone with Israeli, Palestinian and other leaders trying to broker an end to the fighting.

He called on both sides to resume negotiations for a ceasefire and negotiations in Cairo “to address the underlying issues.”

“This madness must stop,” Ban said.