Officials on the poor Indian Ocean island nation had earlier this week reported 74 fatalities and 805 cases.
WHO's director for health emergencies in Africa, Ibrahima Soce Fall, told reporters in Geneva that out of 1,153 suspected cases, 300 had been laboratory confirmed.
Fall said WHO has sent 1.3 million doses of antibiotics to Madagascar, enough to treat 5,000 patients and protect another 100,000 people who may have been exposed to the infectious disease.
Madagascar has suffered plague outbreaks almost every year since 1980 - typically between September and April.
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The current outbreak is unusual as it has affected urban areas - especially the capital Antananarivo - increasing the risk of transmission and sparking panic in the population.
Plague bacteria develops in rats and is carried by fleas.
In humans, the pneumonic version is transferred through coughing and can be fatal within 72 hours.
Most of the victims recorded in Madagascar have been infected with the pneumonic form. The bubonic form is less dangerous.
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