The medical aid group Doctors Without Borders says lead poisoning has killed some 400 children in northern Nigeria over the past six months.
The new toll is more than double the 160 deaths, including 111 children, that Nigerian authorities reported in June.
At that time, the World Health Organization said lead concentrations in parts of Nigeria's Zamfara state were 250 times higher than those allowed in residential areas in the United States and France.
Nigerian health officials have said the poisoning is linked to illegal gold mining.
The WHO has sent epidemiologists and pediatricians to help contain the outbreak and prevent similar problems in the future.
Concentration of lead in the body can damage the kidneys, nervous system and reproductive system. Children under the age of six are especially vulnerable.
Some information for this report was provided by AFP and Reuters.