More than 160 children have died of lead poisoning in Nigeria in recent weeks, health officials say.
The number has been rising since March, when residents started digging illegally for gold in areas with high concentrations of lead.
A total of 163 out of 355 cases of poisoning have proved fatal, a Nigerian health ministry official told Reuters.
The health ministry's chief epidemiologist, Dr Henry Akpan, says: "[The victims] were digging for gold, but the areas also have large concentrations of lead."
Health authorities have set up two camps in the area to treat people who are suffering symptoms of lead poisoning.
Virtually no children in villages
The BBC reports that the deaths were discovered during the country's annual vaccination programme, when officials realised there were virtually no children in several remote villages in the northern state of Zamfara.
Villagers said the children had died of malaria and it was only when a team from international aid agency Medecins Sans Frontieres took blood tests from local people that the high concentrations of lead were discovered.
Zamfara State recently employed a Chinese company to mine gold in the area, the BBC's correspondent says, but villagers have also sought to capitalise by digging for gold themselves - an illegal activity in Nigeria.