Aid group Medecins Sans Frontieres (MSF) said Thursday that at least 18 people had been killed in fighting in South Sudan, including two of its local staff members.

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

"People came to the PoC (Protection of Civilian camp) looking for protection and this should be a sanctuary respected by all parties," he added after the fighting in the northeast town of Malakal.

Initial fighting went on for about three hours, forcing around 600 people - mostly women and children - to gather inside the MSF hospital, said the MSF statement.

"MSF teams worked through the night to treat the injured. At least 25 of the initial intake of patients to the hospital had suffered gunshot wounds and eight of them required surgery," it said.

The two MSF staff killed were South Sudanese employees, who were attacked in their own homes. "Our thoughts are with the families of our colleagues who have lost their lives," said the MSF.

Earlier UN Secretary-General Ban Ki-moon said that an outbreak of fighting at a UN peacekeeping base sheltering civilians in South Sudan has killed at least seven people and injured 40.

Violence between the ethnic Dinka and Shilluk communities broke out overnight and continued into the day, he said.

The MSF statement added that some 40,000 people are currently sheltering in the Malakal PoC site, many from areas where no aid had been available for months.

Source :AFP