The WHO has sent a rapid response team to South Sudan to investigate a mysterious illness that has so far killed 89 people.

The country's health ministry reported that an unknown disease has killed dozens in a town north of Jonglei state. A task force was sent to the area, one of the worst to be hit by recent flooding, to collect samples from the sick.

"We decided to send a rapid response team to go and do risk assessment and investigation; that is when they will be able to collect samples from the sick people - but provisionally the figure that we got was that there were 89 deaths," said the WHO's Sheila Baya.

The samples that have so far been collected have tested negative for cholera.

Upwards of 700,000 people have been affected by the worst flood to hit the country in 60 years, the UN refugee agency UNHCR said, linking the phenomenon to climate change.  

Health facilities are under severe pressure, as the floods have caused food shortages and illnesses, putting pressure on the healthcare system. "We are extremely concerned about malnutrition, with severe acute malnutrition levels two times the WHO threshold, and the number of children admitted to our hospital with severe malnutrition doubling since the start of the floods," Médecins sans frontières said.

Nearly all of South Sudan's population depends on international food aid, with the country also depending on health and education services provided by UN agencies and aid groups.