"Nigeria's Lassa fever outbreak has reached record highs, with 317 laboratory confirmed cases," the WHO said in a statement, quoting figures from the Nigeria Centre for Disease Control (NCDC).
Lassa fever, named after the town in northern Nigeria where it was first identified in 1969, is endemic but the number of confirmed cases "has never reached" this level before, it added.
On February 6, 31 people were reported to have died from the disease, which is caused by a virus of the same family as Marburg and Ebola.
Fourteen health workers have been affected by the virus in six states.
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The WHO said a total of 2,845 people who have come into contact with patients have been identified and were being monitored.
Worst-hit by the virus is Edo state, in the south, where Crusoe Osagie, a media aide to the state government, said mostly rural areas were affected.
"Edo state is the only state with Lagos to have a special treating hospital for this disease, which is in Irrua," he told AFP. "People come here from all over. "
WHO said it was supporting the NCDC effort to contain the disease and treat those affected in treatment centres across the country.
"The ability to rapidly detect cases of infection in the community and refer them early for treatment improves patients' chances of survival and is critical to this response," said the world body's representative in Nigeria, Wondimagegnehu Alemu.
Lassa fever is spread through contact with food or items contaminated with rats' urine or faeces or after coming into direct contact with the bodily fluids of an infected person.
More than 100 people were killed in 2016 in an outbreaks affecting 14 of the 36 states, including Lagos and the capital Abuja.
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