At least 18 people have died of cholera in war-torn South Sudan, the health ministry said Tuesday, as it declared an outbreak of the highly contagious infection.

At least 18 people have died of cholera in war-torn South Sudan, the health ministry said Tuesday, as it declared an outbreak of the highly contagious infection.

Minister of Health Riek Gai Kok told reporters that 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.

At least 167 people died with over 6,400 cases reported in an outbreak last year, which was later contained.

"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," he added.

Stamping out cholera -- transmitted through drinking water or eating food contaminated with feces 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.

Civil war began in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.