A further 764 are suspected to be infected, and 2,845 contacts have been identified.
On average, Lassa fever is deadly in 1% of all individuals infected, with higher rates of 15% morbidity among people hospitalized for the illness. As of Sunday, the case fatality ratio was 22%, according to the Nigeria Centre for Disease Control.
Although it's endemic to the country, Lassa fever numbers have never reached this proportion before, according to the WHO.
Nigeria's Centre for Disease Control said Wednesday that it was facing an "unprecedented outbreak" that has spread to 18 states since it began in January.
The disease can cause fever and hemorrhaging of various parts of the body -- including the eyes and nose -- and can be spread through contact with an infected rat.
Person-to-person transmission is low but has been seen in Nigerian hospital settings this year. Fourteen health workers were infected, of whom four died within eight weeks.
The WHO said Wednesday that health facilities were overstretched in the southern states of Edo, Ondo and Ebonyi, and it is working with national reference hospitals and the Alliance for International Medical Action to rapidly expand and better equip treatment centers.
It also hopes to reduce further infections to hospital staff.
"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 Dr. Wondimagegnehu Alemu, the WHO representative to Nigeria.
State health authorities are mobilizing doctors and nurses to work in treatment centers. Four UK researchers have also been deployed to Nigeria to help control the unusually large outbreak.
"Given the large number of states affected, many people will seek treatment in health facilities that are not appropriately prepared to care for Lassa fever," Alemu said.
"The risk of infection to health care workers is likely to increase."
Lassa fever is endemic in most of West Africa, especially Nigeria, where it was discovered in 1969.
Edo, Ondo and Ebonyi states account for 85% of cases, said Dr. Chikwe Ihekweazu, director of the Nigeria Centre for Disease Control, in a statement.
According to the US Centers for Disease Control and Prevention, touching, eating or sniffing foods and other household items that have been contaminated by multimammate rat feces or urine can aid transmission.
Nigerians, especially in those three states, should "continue focusing on prevention by ensuring they prevent access to their foodstuff by rodents," Ihekweazu said.
There is risk of the virus spreading to other West African countries due to increased migration, said Dr. Oyewale Tomori, professor of virology at Redeemer's University in Nigeria and the former regional virologist for the WHO's Africa Region.
"There is always cause for alarm in West Africa, where the rodent host of can be found in virtually all countries of West Africa," he said.
Benin, Liberia and Sierra Leone have all reported cases of Lassa fever over the past month, according to the WHO, but risk of further international transmission is low for now.