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A UN official warned on Monday that "an even more hellish scenario" looms in Gaza in which humanitarian aid simply grinds to a halt.

"The conditions required to deliver aid to the people of Gaza do not exist," said Lynn Hastings, UN 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 US 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 UN teams and trucks, Hastings said.

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

The spokesman for UN Secretary-General Antonio Guterres meantime called again for "a sustained humanitarian ceasefire 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 Hamas militants, almost two months after the start of the war triggered on October 7 by a bloody attack by the Islamist movement on Israel.