NAIROBI (Reuters) - Underground water supplies that 1.5 million Palestinians rely on for drinking and farming could collapse due to years of over-use and contamination worsened by conflict, the United Nations said on Monday.

A report launched by the U.N. Environment Programme (UNEP) at its headquarters in Kenya said the hostilities earlier this year in the Gaza Strip meant the aquifer below it needed to be “rested” and alternative water sources found.

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

It said the water supplies were being affected by salt water intrusion, as well as by pollution from sewage and agricultural run-off, and that pollution levels were so bad that Gaza infants were now at risk from nitrate poisoning.

UNEP estimates that well over $1.5 billion may be needed over 20 years to restore the aquifer to health, including building desalination plants.

UNEP’s report said last year’s 22-day conflict with Israel had generated some 600,000 tonnes of demolition debris, and that an estimated 17 percent of cultivated land, including greenhouses and orchards, had been severely affected.

Also having an impact were sewage spills after power cuts to treatment facilities, some of which probably percolated through the Gaza Strip’s porous soils into ground water, and an increase in the build-up of hazardous hospital waste at landfill sites.