Antonio Guterres directly warns Security Council member states that the humanitarian system in Gaza may collapse.
In a dramatic move, the UN chief invokes a UN article not used in decades to call on members to "avert a humanitarian catastrophe".
He says he expects public order to completely break down because of continuing Israeli bombardment and lack of essential supplies.
Epidemics could spread and pressure rise for mass displacement into neighbouring countries, he says.
Israel agrees to allow a "minimal" amount of additional fuel into Gaza to prevent humanitarian collapse and the outbreak of epidemics.
Legislation to provide new security assistance for Israel has been blocked by Republican members of the US Senate.
Hamas attacked Israel in October, killing 1,200 people and taking 240 hostages, some of whom were released during a short-lived truce.
Hamas officials in Gaza say Israel has killed more than 16,200 people in its retaliatory campaign, including about 7,000 children.
BDST: 0845 HRS, NOV 28, 2023