Two days after donor states pledged USD 5.4 billion to rebuild the Gaza Strip, Ban toured some of the areas worst hit during the July-August conflict between Israel and the territory's Hamas rulers.
"No amount of (UN) Security Council sessions, reports or briefings could have prepared me for what I witnessed today," he said after being driven through the ruins of Gaza City's Shejaiya district and the nearby Jabaliya refugee camp.
"The shelling of the United Nations school is absolutely unacceptable. These actions must be fully and independently investigated," he said.
Relatives of the dead held up posters showing their loved ones and disabled casualties waited to see Ban.
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The UN chief also called on Palestinian militant groups to cease firing rockets at Israel from the territory.
"I repeat here in Gaza the rockets fired by Hamas and other military groups, must end. They have brought nothing but suffering," he said.
After meeting in Gaza City with members of a new Palestinian consensus government, Ban told reporters the devastation he had seen was far worse than that caused in the previous Israel-Gaza conflict of winter 2008-2009.
"This is a much more serious destruction than what I saw in 2009," he said.
Ban, who last visited the territory in 2012, said at a donor conference in Egypt on Sunday his trip to the Palestinian enclave was "to listen directly to the people of Gaza".
Donations include USD 1 billion from Qatar, USD 212 million from the US and 450 million euros from the EU.