The United Nations says South Sudan's cholera outbreak has reached the country's second-largest city, as the number of cases nationwide rises beyond 5,500.

The U.N. humanitarian agency said Friday that cases of the sometimes fatal gastrointestinal disease have been confirmed in Malakal, which has been ravaged by the country's three-year civil war.

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The U.N. says cholera now has been confirmed in 14 South Sudan counties, with at least 137 deaths since June.

Cholera cases have been reported in the two counties recently declared to be in the grip of famine, Mayendit and Leer.

Many of the country's cholera cases have been reported around the capital, Juba.

Cholera is a fast-developing, highly contagious infection that can spread in areas without clean drinking water and with poor sanitation.