UN health experts say they remain mystified by an unidentified disease in Angola that has killed four people.
About 200 other victims are in hospital suffering from symptoms which include fever and extreme drowsiness.
A team of specialists has spent two weeks studying the outbreak at a clinic in a northern suburb of the capital.
The World Health Organization said it was important "people do not panic" and report to hospital if they have such symptoms, AFP news agency reports.
A WHO spokeswoman said experts were still carrying out tests.
"More and more people have fallen ill but we have not been able to identify the cause of their sickness," Reuters news agency quotes Fadela Chaib as saying at a news briefing.
"It might be exposure to a toxic substance, something in the environment. But the possibility of an infectious disease is not yet excluded."
The outbreak in Luanda's northern Cacuaco suburb was first reported in October.