Hundreds of thousands of displaced Gazans face flooding of their tents and shelters by heavy rains, and materials for shelters and sandbags are not being allowed to enter the enclave, the UN International Organization for Migration said.
Nearly 800,000 displaced people are at heightened risk of potentially dangerous flooding in low-lying, rubble-filled areas where families are living in unsafe shelters, the IOM said.
Insufficient drainage and waste management also heighten the risk of disease outbreak.
According to the IOM, materials to help reinforce shelters, such as timber and plywood, as well as sandbags and water pumps to help with flooding, have been delayed from entering Gaza due to ongoing access restrictions.
Israel said it is meeting its obligations and accuses agencies of inefficiency and failing to prevent theft by Hamas.
COGAT, the Defense Ministry body that oversees humanitarian matters, was not immediately available for comment.