Swine flu has killed seven members of an endangered Amazonian tribe, an indigenous rights organisation says, according to BBC.
Survival International said several hundred members of the Yanomami tribe in Venezuela could be infected.
The Venezuelan government has yet to confirm the deaths but said that a team was in the region to investigate.
An outbreak among the isolated tribes of the Amazon could spread among the indigenous population very quickly and kill many, campaigners fear.
Survival International, a London-based organisation, says that is already happening among the Yanomami in the border region between Venezuela and Brazil.
The organisation`s director, Stephen Corry, says the situation is "critical" and is calling for Venezuela and Brazil to take immediate action to halt the epidemic.
They also needed to radically improve the Yanomami`s access to healthcare, he said.
A member of the regional government`s medical team told the BBC swine flu was the suspected cause of the deaths of a pregnant woman and three small children.
The Yanomami have been hurt by epidemics in the past, particularly when influenza and malaria were brought by miners in the 1980s.
Survival International estimate that as much as a fifth of the community was killed during that period and that the Yanomami population has fallen to about 32,000.