18 people were killed in the violence that broke out in the Malakal Internally Displaced Persons, IDPs camp, says International Organization for Migration, IOM.
It says casualty figures may increase once humanitarian workers gain access to the affected areas of the site to assess the situation, Radio Bakhita reports.
IOM adds that they and other medical non-governmental organizations are treating 91 people injured during the clashes.
Violence involving the use of small arms, machetes and other weapons broke out between Shilluk and Dinka youth in the UN Protection of Civilians site in Malakal on Wednesday.
The United Nations Mission in South Sudan in a statement strongly condemned the violence.
UNMISS protects over 47,000 civilians in Malakal.
It is protecting 198,440 civilians at six bases throughout South Sudan.