The United States said Thursday there was little doubt Israeli artillery was the source of a “totally indefensible” strike that killed 16 people at a UN school in Gaza.

White House spokesman Josh Earnest noted that UN Secretary-General Ban Ki-moon had said all available evidence pointed to Israeli artillery and that the Israeli government had acknowledged its forces were firing in the area in response to Hamas fire near the school.

“So while we underscore the importance of a full and prompt investigation of this tragic incident, as well as a shelling of other UN facilities and schools that have been hit, it does not appear there’s a lot of doubt about whose artillery was involved in this incident,” he said.

“That is why we have continued to urge Israeli military officials to live up to their high standards that they have set for the protection of innocent civilians. There is clearly more that can and should be done to ensure the safety of innocent civilians.”

An estimated 3,300 civilians had taken shelter in the UN school in northern Gaza when it was hit Wednesday.

“The shelling of a UN facility that is housing innocent civilians who are fleeing violence is totally unacceptable and totally indefensible,” Earnest said.

The attack had previously drawn a sharp condemnation by Ban who called it “reprehensible.”

Despite the mounting concern over the high civilian casualties, Prime Minister Benjamin Netanyahu has vowed to press ahead with an offensive to curb rocket fire destroy a network of cross-border tunnels that Hamas has built to attack Israel.

Civilians have accounted for two thirds of the 1,397 people killed in 24 days of fighting, according to UN estimates. Israel contests the figures and says hundreds are combatants.