UN: Israel must allow full access for aid, supplies to rehabilitate Gaza

"Given the scale and nature of the damage and loss of life, there are also obvious concerns about a lack of wider respect for international humanitarian law, particularly the principles of distinction and proportionality. There must be accountability," he said.

Holmes cited the toll of the conflict: some 1,300 Palestinians killed, and more than 5,300 injured, 34 percent of them children, according to Palestinian Ministry of Health figures that have not been seriously challenged; 21,000 homes destroyed or badly damaged; over 50,000 people displaced in UN Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) structures during the height of the fighting, with tens of thousands more sheltering with families and friends.

"Widespread destruction was caused to Gaza's economic and civil infrastructure," he said. "I saw for example, an entire industrial and residential area in East Jabalia which had been systematically bulldozed, an area of at least one square kilometer; one of the best schools in Gaza reduced to rubble; and much of the Al Quds hospital in Gaza City burned out."

But he stressed the critical need to look forward to bring urgent relief to Gaza after 18 months of closure, which steadily weakened health, livelihoods and infrastructure even before the recent offensive.

"A massive humanitarian effort is now needed in areas such as food security, nutrition, water and sanitation, shelter, essential repairs of power, roads and other basic infrastructure, rebuilding the health system, rubble removal, unexploded ordnance and psychosocial care. As only one example, 1.3 million Gazans, almost90 percent of the population, now need food aid," he said, noting that he would launch a Flash Appeal on Feb. 2.