A cholera outbreak has sickened more than 540 people in Kenya's largest refugee camp and killed seven in the last few weeks, the medical charity Medecins Sans Frontieres (MSF) said, warning the epidemic could worsen due to heavy rains.

MSF has set up a cholera treatment centre in the sprawling Dadaab camp, which is home to some 350,000 refugees, mostly Somalis and South Sudanese who have fled neighbouring civil wars.

"The rains are exacerbating an already precarious hygiene situation," Charles Gaudry, the head of MSF in Kenya, said in a statement. "After each heavy rain, we see an increase of patients in our treatment centre."

More than 8,000 people have been treated for cholera across Kenya since December 2014.

Cholera, which spreads through contaminated food and drinking water, causes diarrhoea, nausea and vomiting. It can be fatal if not treated, though most patients recover if treated promptly with oral rehydration therapy.

It spreads easily in crowded camps with poor sanitation like Dadaab, where most refugees live under tarpaulins.

It is hard for the refugees to maintain good hygiene because of shortages of latrines and water, MSF said.

Regular hand-washing is one of the best ways to prevent the spread of cholera, but the refugees have not received soap for the last two months, it said.

"The disease can only be halted by improving hygiene conditions," MSF said.

Funding shortages have forced the United Nations to cut food rations to Dadaab twice since 2013.