Addressing a Cairo summit as the conflict raged into its third week, Guterres said the Palestinian enclave of 2.4 million people was living through "a humanitarian catastrophe" with thousands dead and more than a million displaced.
"We meet in the heart of a region that is reeling in pain and one step from the precipice," he told the meeting that included the leaders of Egypt, Iraq, Jordan and the United Arab Emirates as well as of Italy and Spain and Palestinian president Mahmud Abbas.
Guterres said "the grievances of the Palestinian people are legitimate and long" after "56 years of occupation with no end in sight" but stressed that "nothing can justify the reprehensible assault by Hamas that terrorised Israeli civilians".
He then stressed that "those abhorrent attacks can never justify the collective punishment of the Palestinian people".