The outbreak is moving quickly, with several British holiday hotspots now deemed at risk of the epidemic spreading, including Seychelles, South Africa and La Reunion.
Mozambique, Tanzania, Kenya, Ethiopia, Comoros and Mauritius are the six other countries to have received the heightened alert.
It has been reported as many as 50 aid workers are believed to have been among the people infected.
The African branch of the WHO states 93 people have lost their lives to the disease so far, lower than the 124 noted in official UN figures.
A WHO official said: "The risk of the disease spreading is high at national level... because it is present in several towns and this is just the start of the outbreak."
However, amid widespread fears it could reach Europe and wreak havoc, the WHO has stressed the overall global risk is considered to be "low".
How many people have died?
The statement questions the UN figures released last week that warned the plague outbreak has infected less than 1,200.
WHO admitted the outbreaks have centered in cities, including the capital of Madagascar, Antananarivo - heightening the risk of it spreading.
Growing concerns
Officials are growing concerned as around two thirds of the cases are suspected to be pneumonic plague, spread through coughing, sneezing or spitting.
It is more deadly then the bubonic variation of the disease which killed a third of Europe's population in the 1300s before being largely wiped out.
Madagascar sees regular outbreaks of the disease, but this one has caused alarm due to how quickly it has spread and a high number of fatalities.
Like the bubonic form that often is found in Madagascar's remote highlands, it can be treated with common antibiotics if caught in time.
This outbreak is the first time the disease has affected the Indian Ocean island's two biggest cities, Antananarivo and Toamasina, officials said.
Around 600 cases are reported each year on the island. But this year's outbreak is expected to dwarf previous ones as it has struck so early.
Drafting in help
International agencies have so far sent more than one million doses of antibiotics to Madagascar. Nearly 20,000 respiratory masks have also been donated.
However, the WHO advises against travel or trade restrictions. It has previously asked for US$5.5 million ($7.9m) to support the plague response.
Despite its guidance, Air Seychelles, one of Madagascar's biggest airlines, stopped flying temporarily earlier in the month to try and curb the spread.
A Foreign Office spokesman previously said: "There is currently an outbreak of pneumonic and bubonic plague in Madagascar."
Outbreaks of plague tend to be seasonal and occur mainly during the rainy season, with around 500 cases reported annually.
