An outbreak of lead poisoning linked to illegal gold mining that left more 160 people dead in Nigeria is unprecedented, the US Centres for Disease Control and Prevention (CDC) said.
More than 100 of those killed in the poisoning in recent weeks are children aged five years or under.
Nigerian health authorities last month roped in the CDC, along with the World Health Organisation (WHO) and Medecins Sans Frontieres(MSF), to help control the epidemic which the government said started in January.
"The scope of the poisoning is unprecedented in CDC's work with lead poisoning worldwide," CDC said in a statement received here from Atlanta.
"This is because of the severity of the poisoning, the number of fatalities, the large number of children and adults with symptomatic poisoning, and the extent of the environmental contamination," it said.
Nigerian health epidemiologists last week reported 111 toddler deaths out of the 163 lives lost.
The epidemic has hit five villages in Nigeria's northwest Zamfara State in recent weeks, affecting more than 300 people.
The poisonings were caused by the illegal extraction of ore by villagers, who would transport crushed rock home from the mines.
They would then extract the gold and haphazardly dispose of the soil containing lead deposits which children would play with.
Junior health minister Suleiman Bello on Monday said the outbreak had been brought under control.
CDC said "some children are still coming into healthcare facilities with severe symptomatic lead poisoning, and there continue to be some deaths."
CDC assisted with "surveying the population about the deaths of children," as well as collection and analysis of blood and soil samples.
It also helped with the environmental cleanup.
"Initial results have found extremely high levels of lead in blood of children and adults and in the soil," it said.