JUBA: Fighting at a United Nations compound sheltering people fleeing conflict in South Sudan has killed 18 people, including two Medicins Sans Frontieres workers, the international medical aid group said.

South Sudan descended into civil war in December 2013 when a row between President Salva Kiir and his sacked deputy Riek Machar ended with fighting that often occurred along ethnic fault lines.

People have been taking refuge in UN-administered "protection of civilian" sites, or POCs, since then. Thousands have been killed and more than 2 million people displaced from their homes since late 2013.

The UN peacekeeping mission, UNMISS, had said on Thursday fighting the night before between youths sheltering in the U.N. compound in Malakal had killed five and wounded 30 after violence erupted between two ethnic groups.

The UN secretary-general's spokesman said later at least seven people had been killed.

"At least 18 people were killed in armed conflict that erupted ... in the Protection of Civilians site in Malakal ... including two South Sudanese staff members of ... Médecins Sans Frontières (MSF) who were attacked in their homes," the medical charity said in a statement.

MSF, also known as Doctors Without Borders, said it had also treated 36 people wounded in the fighting, including at least 25 with gunshot wounds.

“This attack on civilians is outrageous and we demand that armed groups stop these actions,” Marcus Bachmann, coordinator of MSF projects in South Sudan, said in the statement.

UNMISS said youths from the Shilluk and Dinka ethnic groups - both staying in its protection site - began the fighting on Wednesday night using small arms, machetes and other weapons.

UNMISS says the Malakal site shelters 47,791 people out of a total 198,440 that live in six of its bases throughout South Sudan.

Kiir and Machar signed an accord last August to end fighting that has killed thousands of people.

The warring parties agreed in January to share ministerial positions in a transitional government of national unity, and earlier this month Kiir re-appointed Machar to his old post as vice president.