Brazil has airlifted 16 starving Yanomami tribal people to receive urgent treatment, after the government declared a medical emergency.
The indigenous people live in a reserve in Brazil's northern state of Roraima.
President Luiz Inácio Lula da Silva has accused his predecessor, far-right Jair Bolsonaro, of committing genocide against the rainforest tribe.
The government declared a medical emergency after hundreds of Yanomami children died from malnutrition.
The deaths are linked to water pollution caused by mining and logging in the densely forested area, where food insecurity is rife.
On Saturday President Lula visited Roraima, which borders Venezuela and Guyana, following reports of severe malnutrition among Yanomami children and said he was "shocked" by what he found.
"More than a humanitarian crisis, what I saw in Roraima was genocide: a premeditated crime against the Yanomami, committed by a government insensitive to suffering," he said later. "I came here to say we are going to treat our indigenous people as human beings."
An estimated 28,000 indigenous people live in the Yanomami reserve. They hunt, practise small-scale slash-and-burn agriculture and live in small, scattered, semi-permanent villages.
In his four years in power, Mr Bolsonaro often criticised the size of the indigenous reserves and promised to open some of them to agriculture and mining. His government weakened environmental protections, and critics said his rhetoric emboldened illegal activity in the region.
Today, some 20,000 illegal miners are estimated to operate inside the Yanomami reserve, which is rich in gold, diamonds and minerals. In 2021, miners in the area opened fire on the Yanomami using automatic weapons.
The new Lula government says more than 500 indigenous children have died in the past few years from drinking water contaminated with mercury, which is directly linked to illegal gold mining.
Lula was sworn in as president on 1 January after narrowly defeating Jair Bolsonaro, and Brazilian society is deeply polarised.
Sonia Guajajara, the Minister of Indigenous Peoples, said: "We must hold the previous government accountable for allowing this situation to get worse to the point where we find adults weighing like children, and children reduced to skin and bones."
The Interior Minister, Flavio Dino, also accused the previous government of abandoning the indigenous community and promised an investigation.
Besides airlifting some of the most seriously ill members of the tribe, Brazilian authorities announced that the health ministry would create a field hospital and send supplies and health professionals to the area.
Dr Andre Siqueira, a specialist in tropical medicine who is currently working in the Roraima region, told the BBC he had come across cases of severe malnutrition in entire indigenous families and said the situation was "catastrophic" and "disastrous".
He said the Yanomami were not the only tribes facing serious threats to their existence, and he had witnessed "similar situations of lack of assistance and care" in other indigenous territories.
"This is something that urgently needs to be addressed, because our humanity depends on it", Dr Siqueira said.