"We discovered unusual cases of abdominal pains with vomiting, nausea and some having convulsions," Akpan told the Reuters news agency.

He said health officials had also found children playing in water close to the mining sites when they visited.

"These people were around the area where they were digging for gold. The fatality rate is 46 per cent."

Containing the 'epidemic'

Nigerian authorities have asked for assistance from various international health agencies, including the World Health Organisation (WHO) and the US Centers for Disease Control and Prevention (CDC), to help contain the outbreak of lead poisoning.

Akpan said the number of cases has fallen since April after local authorities halted illegal mining and began evacuating residents.

"We are doing a lot to contain the epidemic," he said.

"We have been able to get on top of this. The number of reported illnesses have fallen. We are winning."

The impoverished state of Zamfara is rich in minerals including gold, copper, iron ore and manganese.

Goodluck Jonathan, Nigeria's president, recently inaugurated a mineral processing plant in the state, which is seeking to attract investment.