"I repeat my call for a humanitarian ceasefire, the unconditional release of all hostages, and the delivery of life-saving supplies," UN Secretary-General Antonio Guterres said in a statement.
"Misery is growing by the minute. Without a fundamental change, the people of Gaza will face an unprecedented avalanche of human suffering."
Before the war, about 500 trucks a day rolled into sealed-off Gaza to bring in supplies but in recent days the average is down to only 12, which enter from the Rafah border crossing with Egypt, he said.
Guterres said the need is now "far greater than at any time before."
Without a change to how aid goes in, the United Nations will not be able to keep delivering assistance, the UN chief said.
"The verification system for the movement of goods through the Rafah crossing must be adjusted to allow many more trucks to enter Gaza without delay," Guterres said.
"The humanitarian system in Gaza is facing a total collapse with unimaginable consequences for more than two million civilians," he warned.
"As the bombing intensifies, needs are growing ever more critical and colossal."
Internet, communications cut across Gaza
The Hamas government said Israel "cut communications and most of the internet" across the Gaza Strip on Friday. The government's media office accused Israel of taking the measure "to perpetrate massacres with bloody retaliatory strikes from the air, land and sea," as heavy strikes hit northern Gaza.
AFP journalists in Gaza confirmed they were only able to communicate in limited areas where they could connect to Israeli networks across the border.
Palestinian telecoms provider Jawwal announced "the total cut of all communication services and internet" in the territory. "The heavy bombardment in the last hour destroyed all remaining international routes connecting Gaza to the outside world," Jawwal wrote on its Facebook page.
Global internet monitor NetBlocks reported "a collapse in connectivity in the Gaza Strip with high impact to Paltel," Jawwal's owner, citing live network data. "The company is the last remaining major operator to supply service as connectivity declines amid ongoing fighting with Israel," NetBlocks wrote on X, formerly Twitter.