An isolated Amazon tribe on the Venezuela-Brazil border has been hit by a measles epidemic that could kill hundreds of indigenous people if emergency action is not taken, NGOs have warned.

The Yanomami people, who live in communities on both sides of the border, have reported up to 100 cases of measles since the first reports of illness began to emerge in March, according to media in Brazil. But with most living far from medical facilities, assessment and treatment is difficult.

Across northern Brazil as a whole, at least 500 people have been confirmed to be infected by measles, while 1,500 suspected cases are also being investigated, according to authorities. Survival International, an NGO working to protect indigenous groups, said the outbreak could prove devastating to the Yanomami, who are not protected from the disease.

There had very likely already been deaths deep in the forest and many more could follow, the group said.

Stephen Corry, the group’s director, said: “When tribal people experience common diseases like measles or flu, which they’ve never known before, many of them die and whole populations can be wiped out. These tribes are the most vulnerable peoples on the planet. Urgent medical care is the only thing standing between these communities and utter devastation.”