Militants attacked a hospital in eastern Congo on Saturday, the latest in a string of assaults on health facilities in the Ebola-hit region.

One militant was killed and four more arrested after the attack on the Katwa hospital in the city of Butembo, said Sylvain Kanyamanda, the mayor of Butembo.

The incident comes one day after at least one health worker was killed in a militant attack on another hospital in Butembo.

An Ebola treatment centre near the hospital hit on Saturday was also the target of an attack in February that led the aid organization Medecins Sans Frontieres to stop its work there.

Responding to Friday's attack, the head of the World Health Organization (WHO), Tedros Adhanom Ghebreyesus, wrote on Twitter. ‘This is a very sad day. Health workers are working around the clock to save lives. Health workers are #NotATarget.’   

The city of Butembo in the province of North Kivu is at the centre of the current Ebola epidemic. It is the second most serious known outbreak of the life-threatening haemorrhagic fever.

Because of repeated attacks on Ebola centres and active militias in the region, aid workers have so far failed to bring the epidemic under control.

In North Kivu and neighbouring Ituri, around 1,300 people have already contracted Ebola, and more than 850 died as a result of the infection.

To stem the outbreak, around 100,000 people have already received an experimental Ebola vaccine in the region.