Aid operations in northwestern Central African Republic (CAR) have yet to resume, four days after an international staff member of the charity Médecins Sans Frontières (MSF) was killed, because the situation there is still worrying, the United Nations said.

"I think the whole aid community was horrified by what happened," Toby Lanzer, the UN Humanitarian Coordinator in CAR said on 15 June. "We are very concerned about the situation in the northwest."

Elsa Serfass, 27, was struck by a bullet while travelling in the Ngaoundai region on 11 June. She was on her first assignment with MSF-France, working as a logistician based in Paoua town. MSF and other NGOs suspended operations and moved staff to the capital, Bangui.

The UN also stopped movement of its staff around Paoua, where tens of thousands of conflict-affected people are in need. "We now have a fairly good indication of how the incident happened, and are reviewing the situation every day," Lanzer told IRIN. "But staff safety comes first."

In a statement, MSF said the future of its activities in the area would depend on the result of discussions with all the parties involved and on the evaluation of its space for work.

"The rebel group, the Popular Army for the Restoration of Democracy [APRD], is known to be active in this region," it noted. "MSF duly informed leaders of this group in advance, as well as the other parties to the conflict, of the team's movements. Elsa and her colleagues were travelling in a vehicle clearly identified with the MSF logo."

According to MSF, the vehicle had driven past several villages when two shots were fired at it. One hit the back left-hand side, fatally wounding the staff member. "The driver stopped immediately, and he and the nurse climbed out of the car, stating clearly: ‘Don't shoot. We are doctors with Médecins Sans Frontières.’ Shortly after, around 20 armed men came out of the bush bordering the road and presented themselves as members of the APRD," MSF said.

The CAR government condemned the incident. A presidential communiqué issued on Thursday called the killing unjustified and urged action against those involved.

The APRD rebels initially denied involvement but later assumed responsibility. "A young rebel mistook the vehicle," a spokesman, Christophe Gazambeti, told reporters. "He believed the vehicle belonged to the presidential guards who were in the area."

In a separate interview, Maj Bertin Wafio of the APRD said: "I am sorry for what happened; I condemned the incident and I hope this won’t happen again."

In May, the Italian charity, Cooperazione Internazionale (COOPI), suspended activities in Bozoum, Ouham Pendé prefecture, after the abduction of two health workers. The NGO works in Bozoum, Bocaranga and Ngaoundai, which together have a population of 150,000-200,000.

According to the UN, nearly 300,000 people have fled their homes in the CAR over the past year because of violence.