Mounds of trash rotting in the heat are piling up close to where displaced people are sheltering in Gaza, a U.N. official said on Friday, raising fears about the further spread of disease.

Hundreds of thousands of Gazans who had fled to southern Gaza earlier in the more than 8-month conflict have been uprooted again since Israel expanded its military operations against Hamas to the southern city of Rafah in early May.

Louise Wateridge, an aid worker with United Nations Palestinian refugee agency (UNRWA), said that a pile of waste weighing an estimated 100,000 tonnes was building up near people's tents in central Gaza.

"It's among the population and it's building up without anywhere to go. It just keeps getting worse. And with the temperatures rising, it's really adding misery to the living conditions here," she told journalists via video link from Gaza.

Israel has refused repeated requests to allow UNRWA to empty the main landfill sites, she said, meaning temporary ones are emerging, she added. Even if permission is granted, Wateridge said UNRWA's humanitarian missions such as trash collection have all but halted due to Israeli refusals to allow fuel imports.

Israel's COGAT, a branch of the military tasked with coordinating aid deliveries into Palestinian territories, did not immediately respond to a request for comment.

Israel, which launched its Gaza military operation after deadly Hamas attacks on southern Israel on Oct. 7, says it has expanded efforts to facilitate aid flows into Gaza and blames aid agencies for distribution problems inside the enclave. It controls fuel shipments into Gaza and has long maintained that there is a risk they are diverted to Hamas.

The World Health Organization's Tarik Jašarević said the trash, along with the rising heat, a lack of clean drinking water and sanitation services, was adding to disease risks.

"It can lead to a number of communicable diseases appearing," he said, mentioning that around 470,000 cases of diarrhea have been reported since the start of the war.

Wateridge, who arrived back in Gaza on Thursday after a four-week absence, said the situation had deteriorated significantly. She described the living conditions as "unbearable" with people sweltering under plastic sheets and cowering in bombed out buildings.