At least 13 people have been killed in a southern region of the Democratic Republic of Congo (DRC) by what aid groups say is an outbreak of the deadly Ebola virus.

Francois Dumont, the spokesman for Médecins Sans Frontières (Doctors Without Borders), said tests had so far confirmed the virus in four people and all of those had survived.

Dumont said it was "highly suspected'' that all 13 fatalities, which occurred in the village of Kaluemba in Western Kasai province, were caused by Ebola virus disease.

However, it is unclear if all the deaths were caused by the disease.

Ebola causes symptoms such as high fever, vomiting and diarrhoea as well as severe internal and external bleeding.

"For sure we are in the middle of an Ebola outbreak,'' Dumont told the Associated Press. 

"But we cannot scientifically confirm the deaths are due to Ebola yet.''

He also said 42 people are suspected to have been infected with the virus, an increase from a previous count of 35 from the Congolese health ministry.

Dumont said MSF was currently monitoring 200 people who have been in contact with those infected, and Doctors Without Borders had set up an isolation unit where one person was recovering.

Last week, the Congolese health minister said laboratory tests had confirmed 11 deaths from the virus in the province, but it is unclear whether those people were included in Dumont's account.

On Tuesday the government of neighbouring Angola said it was closing part of its border with DR Congo to prevent the spread of the outbreak.

Ebola kills up to 90 per cent of the people it infects and is spread through direct contact with the blood or secretions of an infected person or with contaminated objects.

Last year, the Ebola virus killed at least 187 people in the same region of DR Congo.

An Ebola outbreak in 1995 killed at least 245 people in the village of Kikwit.