WHO spokesperson Christian Lindmeier warned of the risk of civilian deaths not directly linked to Israeli bombardment.
"It's an imminent public health catastrophe that looms with the mass displacement, the overcrowding, the damage to water and sanitation infrastructure," Lindmeier told reporters.
Gaza health authorities say more than 8,300 Palestinians have been killed since Israel began air strikes on the Hamas-run enclave in response to the October 7 attacks when Hamas killed 1,400 people in Israel and took more than 200 hostages.
The Israeli military began ground operations in Gaza last week.
Asked if people were dying from complications other than those from the bombardment, Lindmeier said: "Indeed they are."
A spokesperson from the U.N. children's agency, UNICEF, James Elder, warned of the risk of infant deaths due to dehydration with water output at 5% of normal levels.
"So child deaths to dehydration, particularly infant deaths due to dehydration, are a growing threat," he said, adding that children were getting sick from drinking salty water.
About 940 children are reported missing in Gaza, he said, with some thought to be stuck beneath the rubble.
The U.N. humanitarian office said in a statement earlier on Tuesday that water supplies to southern Gaza came to a halt on October 30 "for unknown reasons."
Lindmeier called for fuel to be allowed into Gaza to allow a desalination plant to operate. Israel has blockaded the Gaza Strip and refuses to allow in fuel supplies, saying it could be used by Hamas for military purposes.