The water authority in the Gaza Strip has urged people to boil their drinking water to avoid the spread of disease.
Gaza has suffered water and power shortages since January
The authority said Israel's blockade had delayed essential supplies, including chlorine, and there was now a risk of water being contaminated.
The authority said the situation could lead to a health disaster to Gaza's 1.5m inhabitants.
UN officials have asked Israel to lift the blockade. Israel says its actions are to counter militant rocket fire.
The Coastal Municipality Water Utility issued the warning in radio and newspaper advertisements, blaming Israel for the absence of equipment and chemicals for treating water.
It said there had been no deliveries of chlorine through the Israel-controlled goods crossing since 21 January.
Israel has tightened its blockade of the Gaza since the militant Hamas movement took control in June 2007.
Further restrictions on everything except humanitarian and medical supplies were imposed in recent weeks in response to continued rocket and mortar attacks on southern Israel.