More than 100 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.
The victims were from several remote villages in the northern state of Zamfara.
A total of 163 out of 355 cases of poisoning have proved fatal, a Nigerian health ministry official told Reuters.
Dr Henry Akpan, the health ministry's chief epidemiologist, said: "[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.
The deaths were discovered during the country's annual immunisation programme, when officials realised there were virtually no children in several remote villages in the northern state, says the BBC's Abdullai Kaura Abubakar in Kaduna.
Villagers said the children had died of malaria and it was only when a team from international aid agency Medecins Sans Frontiers took blood tests from local people that the high concentrations of lead were discovered.
Zamfara State had recently employed a Chinese company to mine gold in the area, adds our correspondent.
But villagers had also attempted to capitalise by digging for the precious metal themselves - an illegal activity in Nigeria.
It is likely locals became sick after lead removed during the process of refining gold ore contaminated local water systems, our correspondent says.