Human Rights Watch has accused Israel of committing "acts of genocide" in Gaza by deliberately depriving Palestinian civilians of adequate access to water.
He says Israel's actions include the deliberate destruction of water and sanitation infrastructure.
The campaign group says this may have led to thousands of deaths, which they say also amounts to "executing a crime against humanity".
Israel refused HRW report as "propaganda".
In a post on XIsrael's foreign ministry spokesman said the group was "again spreading the blood label... The truth is the complete opposite of HRW's lies".
The 179-page report states that "since October 2023, the Israeli authorities have deliberately blocked Palestinian access to the amount of water needed to survive in the Gaza Strip".
It says Israel deliberately damaged infrastructure, including solar panels powering treatment plants, a reservoir, and a spare parts warehouse, while time blocking fuel for generators.
He says Israel also cut electricity supplies, attacked repairmen and blocked the entry of repair materials into Gaza.
"This is not just negligence," said HRW's executive director, Tirana Hassan. "It is a calculated policy of deprivation that has led to thousands of deaths from dehydration and disease that is nothing short of the crime of -anti-humanitarian extermination, and an act of genocide.”
The report is based on interviews with dozens of Palestinians from Gaza, including water authority officials, sanitation experts and health care workers, as well as satellite images and data from October 2023 to September 2024.
Israel launched a major offensive in Gaza after Hamas attacked Israel on October 7, 2023, killing around 1,200 people and taking 251 others hostage.
At least 45,129 people have been killed in Gaza since the offensive began, according to the Hamas-run region's health ministry. It does not put a figure on the number who died due to lack of access to water or other such reasons.
The HRW report notes that to constitute the crime of genocide, alleged acts require evidence of intent. He says the findings, including statements from senior Israeli officials, "could indicate such intent".
However, rejecting HRW's accusations against X, Israeli foreign ministry spokesman Oren Marmorstein said Israel had "facilitated the continuous flow of water and humanitarian aid into Gaza, despite operating under constant attacks from the terrorist group Hamas”.
He said that water pipes and pumping and desalination facilities were still working, and that water tankers had been delivering supplies into Gaza again through Israeli routes.
"This report is full of lies that are appalling even compared to HRW's already low ratings," he said.
The HRW report is the latest in a series of accusations by rights groups and others that Israel is committing genocide in its operation in Gaza.
The International Court of Justice (ICJ) - the main court of the UN - is also currently investigating a case raised by South Africa accusing Israel of genocide.
The 1948 Genocide Convention, which was passed in the wake of the Nazi Holocaust of European Jewry, defines genocide as "acts committed with intent to destroy, in whole or in part, a national, ethnic, racial group or religious”.
Israel has vehemently denied the allegations as "completely baseless" and motivated by antisemitism. He says he did not intentionally harm civilians in Gaza, and that he is fighting only against Hamas.