At least 28 children under the age of five have been killed by drinking stream water contaminated with lead in Nigeria's Niger state, health minister Fidelis Nwankwo said, and officials blamed the contamination on illegal mining.

Villagers "were mining for gold, and (the water was made toxic) by its impurities," said Mohammed Usman, a government health official in Niger state on Friday.

Nwankwo said on Thursday that 28 out of 65 children who became ill from the contamination had died, according to reports up to May 12 from villages in the Rafi local government area that borders Kaduna state.

Many cows, goats and chickens and other livestock were killed, too.

"The villagers initially ascribed the infections to witchcraft, but it must have being going for years," Usman said, adding reports suggested the deaths were in April.

"The affected children were found to have high serum lead levels ... 17 to 22 times higher than the acceptable limits as established by the World Health Organization," said Nwankwo, the minister of state for health.

Large-scale lead poisoning from illegal mining has been reported before in Nigeria, notably in Zamfara state in 2010 when at least 163 people died out of 355 who became sick from the toxins.