"Human Rights Watch is once more spreading its blood libels in order to promote its anti-Israel propoganda," the Israeli foreign ministry said in a statement.
The HRW report detailed what the group said was the intentional damaging of water and sanitation infrastructure, including solar panels powering treatment plants, a reservoir and a spare parts warehouse, as well as the blocking of fuel for generators.
Israel also cut electricity supplies, attacked repair workers and blocked the importation of repair materials, it said.
The report concluded that in doing so, "Israeli authorities intentionally inflicted on the Palestinian population in Gaza 'conditions of life calculated to bring about its physical destruction in whole or in part.'"
This, it said, amounted to the war crime of "extermination" and to "acts of genocide".
The HRW report, drawn up over nearly a year, is based on interviews with dozens of Gazans, staff at water and sanitation facilities, medics and aid workers, as well as satellite imagery, photographs, videos and data analysis.
It said Israeli authorities did not reply to requests for information.
The lack of water left Gazans vulnerable to water-borne diseases and complications, such as infected wounds and the inability to heal due to dehydration, HRW said.
Deaths from such cases "are likely vastly underreported", the report said.
Doctors and nurses told HRW "that many of their patients have died from preventable diseases and infections, and healable wounds, due to dehydration and the unavailability of water".
One emergency room nurse cited in the report said they were forced to decide "not to resuscitate children who were severely malnourished and dehydrated".
The rights group called on the international community to "take all measures within their power to prevent genocide by Israeli authorities in Gaza".
That included "discontinuing any military assistance and arms sales or transfers, imposing targeted sanctions, and reviewing bilateral deals and diplomatic relations".
Since October 2023, Israel's attacks in Gaza have killed over 45,000 people and left most of the enclave an unlivable ruin, with survivors on the brink of starvation.
