South Sudan's health ministry has confirmed 18 people have died following an outbreak of cholera.

At least 18 people have died of cholera in war-torn South Sudan, the health ministry says.

Minister of Health Riek Gai Kok told reporters 171 cholera cases have been confirmed in the outbreak, which began in crowded United Nations bases in the capital Juba and spread elsewhere in the city.

Tens of thousands of people have sought shelter in the UN camps during 18 months of civil war.

"After subjecting cases to laboratory tests it was confirmed that there is a cholera outbreak," Riek Gai Kok said, adding that the first case recorded was on June 1.

"The confirmation of the cases automatically triggers the activation of a full-scale response."

Stamping out cholera - transmitted through drinking water or eating food contaminated with faeces or dirty hands - poses an additional major challenge for the government and aid workers.

More than two million people been forced to flee during the war, with over 137,000 civilians sheltering in UN peacekeeping bases, including over 34,000 civilians crammed into camps in the capital alone.

Two-thirds of the country's 12 million people need aid, with 4.5 million people facing severe food insecurity, according to the UN.