BAGHDAD — The World Health Organizations said Tuesday that laboratory tests completed after a shocking fish die-off in Iraq's Euphrates River show the water is contaminated with high levels of bacteria, heavy metals, and ammonia.

Fishermen were stunned to find that hundreds of tons of carp had died suddenly in their river cages earlier this month in a discovery that stirred national anxieties about water pollution. Carp is a staple of the Iraqi diet.

The WHO said the river pollution was toxic for fish but posed no threat to humans. The agency said it tested water samples in its Amman laboratory in response to a request by Iraq's Ministry of Health and Environment.

It said it was conducting a second investigation to see whether a viral infection killed the fish.