Speaking at the World Economic Forum in Davos, Guterres said the warring parties were "ignoring international law, trampling on the Geneva Conventions, and even violating the United Nations Charter".
"The world is standing by as civilians, mostly women and children, are killed, maimed, bombarded, forced from their homes and denied access to humanitarian aid," he said.
"I repeat my call for an immediate humanitarian ceasefire in Gaza, and a process that leads to sustained peace for Israelis and Palestinians, based on a two-state solution."
Israeli Prime Minister Benjamin Netanyahu has rejected calls for a ceasefire, saying Israel will continue its offensive in Gaza until it defeats Hamas and recovers the hostages taken there during a deadly rampage by Hamas gunmen on Oct. 7.
The Israeli bombardment and ground offensive in the Palestinian enclave in response to the Hamas attacks has driven most of Gaza's 2.3 million people from their homes.
It has also caused a grave humanitarian crisis as deliveries of food, fuel and medical supplies have been severely restricted.
The Israeli military recently focused its offensive on the southern end of the enclave, where nearly 2 million people are sheltering in tents and other temporary accommodation, after the initial phase of the war centred on the north.
