JERUSALEM – The Gaza Strip’s underground water supplies are “in danger of collapse” following years of overuse and a devastating war Israel waged in the territory at the turn of the year, the U.N. said Monday.

“The underground water supplies, upon which 1.5 million Palestinians depend for agriculture and drinking water, are in danger of collapse,” the U.N. Environment Program (UNEP) said in a statement accompanying a new report.

“Unless the trend is reversed now, damage could take centuries to reverse. Since the aquifer is a continuum with Israel and Egypt, such action must be coordinated with these countries,” the report says.

Israel and Egypt have sealed off the impoverished territory to all but basic goods since the Islamist Hamas movement seized control in June 2007, severely hampering the upkeep of basic infrastructure.

The sewage system has been particularly hard-hit, as Israel does not allow the import of virtually any pipes or other metal equipment that it fears could be used by Palestinian militants to construct improvised rockets.

The U.N. report estimates that restoring the aquifer beneath Gaza could require $1.5 billion over 20 years, including the construction of desalination plants to take pressure off underground sources.

The report said overextraction was causing seawater to seep into the freshwater aquifer, sending salinity levels across the territory above the 250 milligrams per liter considered safe by the World Health Organisation (WHO).

The U.N. researchers also found high levels of nitrates that exceeded WHO guidelines of 50 milligrams per liter, with one site rising to 331.

High nitrate concentrations in water have been linked to a form of anaemia known as “blue baby syndrome,” the report said.

The report also expressed concern about the state of Gaza’s landfills, saying it found large amounts of exposed hazardous medical waste “in part as a result of an increased level of casualties” during the war.

More than 1,400 Palestinians and 13 Israelis were killed in Dec. and Jan. when Israel carried out a massive military offensive on the territory in a bid to stem Palestinian rocket attacks.