At least 40 people were killed in an outbreak of plague in Madagascar, a media report said on Saturday.

The disease may spread rapidly as fleas on the island have been found with high levels of resistance to a leading insecticide, BBC reported citing a World Health Organisation (WHO) warning.

Humans usually develop the bubonic form of the plague (swollen lymph nodes especially occur in the armpit and groin in persons suffering from bubonic plague) after being bitten by an infected flea carried by rodents, the report said.

If diagnosed early, bubonic plague can be treated with antibiotics.

Two percent of all the cases reported in Madagascar were found to be the more dangerous pneumonic form of the plague, which can be spread person-to-person by coughing, according to WHO.

The first known case was a man hailing from the Tsiroanomandidy district, about 200 km west of the capital Antananarivo during the end of August.

There have now been two confirmed cases in the capital city, including one death.

"There is now a risk of a rapid spread of the disease due to the city's high population density and the weakness of the health care system," the WHO said.

A task force has been formed to manage the outbreak.