The UN says more than 80,000 people have fled the southern Gaza city of Rafah since Monday, as Israeli tanks reportedly gather close to built-up areas amid constant bombardment.
The UN also warned that food and fuel were running out because it was not receiving aid through nearby crossings.
Israeli troops took control and closed the Rafah crossing with Egypt at the start of their operation, while the UN said it was too dangerous for its staff and lorries to reach the reopened Kerem Shalom crossing with Israel.
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After seven months of war in Gaza, Israel has insisted victory is impossible without taking the city and eliminating the last remaining Hamas battalions.
With more than a million displaced Palestinians sheltering there, the UN and Western powers have warned that an all-out assault could lead to mass civilian casualties and a humanitarian catastrophe.
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