The United Nations chief said that Palestinians were enduring "the cruelest phase" of the war in Gaza, where more than a dozen food trucks were looted following the partial easing of a lengthy Israeli blockade.
Aid was just beginning to trickle back into the war-torn territory after Israel announced it would allow limited shipments to resume as it pressed a newly expanded offensive aimed at destroying Hamas.
UN chief Antonio Guterres said "Palestinians in Gaza are enduring what may be the cruelest phase of this cruel conflict," adding that Israel "must agree to allow and facilitate" humanitarian deliveries.
He pointed to snags, however, noting that of the nearly 400 trucks cleared to enter Gaza in recent days, only 115 were able to be collected.
"In any case, all the aid authorised until now amounts to a teaspoon of aid when a flood of assistance is required," he added in a statement.
"Meanwhile, the Israeli military offensive is intensifying with atrocious levels of death and destruction," he said.
Agence France-Presse