With the delivery of humanitarian aid grinding to a halt, a U.N. official has warned that Gaza faced "an even more hellish scenario" in the coming days.

"The conditions required to deliver aid to the people of Gaza do not exist," said Lynn Hastings, U.N. humanitarian coordinator for the Palestinian territories.

Since the end of a seven-day truce, Israeli forces have pushed into southern Gaza, "forcing tens of thousands ... into increasingly compressed spaces, desperate to find food, water, shelter and safety," Hastings said.

"Nowhere is safe in Gaza and there is nowhere left to go."

"If possible, an even more hellish scenario is about to unfold, one in which humanitarian operations may not be able to respond," Hastings said in a statement.

Hastings, a Canadian, rejected the idea of "safe zones" urged upon Israel by the U.S. government where people are still unable to move about freely.

"These zones cannot be safe nor humanitarian when unilaterally declared," she said.

"What we see today," Hastings added, "are shelters with no capacity, a health system on its knees, a lack of clean drinking water, no proper sanitation and poor nutrition for people already mentally and physically exhausted: a textbook formula for epidemics and a public health disaster."

Further complicating aid deliveries, two major roads in Gaza have been declared off-limits to U.N. teams and trucks, Hastings said.

Hastings has her base in Jerusalem but Israel last week informed the U.N. that it would not renew her visa, accusing her of not being "impartial."

The spokesman for U.N. Secretary-General Antonio Guterres, in the meantime, called again for "a sustained humanitarian cease-fire in Gaza and the unconditional and immediate release of all remaining hostages."

The Israeli army has tightened its grip on southern Gaza, where dozens of tanks entered Monday as part of its offensive against the Palestinian resistance group Hamas, in response to its Oct. 7 incursion.