The outbreak of both bubonic plague, which is spread by infected rats via flea bites, and pneumonic plague, spread person to person, has infected 133 people, killing 24 of them, since August 1, according to WHO numbers.
Madagascar has suffered plague outbreaks almost every year since 1980, often caused by rats fleeing forest fires.
But WHO spokesman Christian Lindmeier told reporters the current outbreak had more potential than most to spread further in Madagascar since it has already affected large urban areas, including the capital Antananarivo.
Pneumonic plague, though rarer on the island, spreads more easily, he said, pointing out that 17 of the deaths recorded so far were due to the respiratory version of the disease.
“The overall risk of further spread at the national level is high,” Lindmeier said.
At the regional level, the risk was “moderate due to frequent flights to neighbouring Indian Ocean islands”, he said.
But the risk of a more international spread of the disease was “low”, he said.
For this reason, Lindmeier said the WHO was for now advising against slapping any travel or trade restrictions on Madagascar.
The plague is a highly infectious disease that has killed millions of people across the world in the past before being largely wiped out.
Pneumonic plague can kill quickly, within 18-24 hours of infection if left untreated, but common antibiotics can cure it if they are given early on.
The WHO stressed that most people can recover from the plague if they are rapidly treated and urged anyone in the affected areas who notice respiratory symptoms to seek treatment, which is being provided for free.