UNITED NATIONS/GAZA — A UN official has warned that "an even more hellish scenario" looms in the Gaza Strip where humanitarian aid simply grinds to a halt, as Israeli forces pressed ahead with their air and ground bombardment.

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

Since the end of a seven-day truce on Friday, 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," she said in a statement.

Hastings rejected the idea of "safe zones" urged upon Israel by the United States, where people are still unable to move about freely.

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