U.N. Secretary-General Ban Ki-moon says he is “appalled” by the destruction in Gaza over the last three weeks. He made the comment during a visit to the region three days after both Israel and Hamas instituted cease-fires.

Ban, who arrived in Gaza on Tuesday, said the Israeli shelling of three United Nations buildings, including two schools, was “outrageous and totally unacceptable.”

The U.N. chief also demanded a full investigation into the attacks in which more than 50 Palestinian civilians were killed while they took refuge inside the buildings. Ban said those responsible must be held accountable.

Calling the recent incident a "collective political failure,” Ban said he will share the results of his Gaza trip with world leaders, including new U.S. President Barack Obama.