Forty-two people are believed to have died from the haemorrhagic fever in Congo’s tenth Ebola outbreak since it was discovered in the 1970s.

In all, there have been 66 cases to date, including 39 confirmed and 27 probable, the health ministry said on Tuesday evening, an increase of nine confirmed cases since Monday.

The outbreak has spread from its epicentre in North Kivu province to neighbouring Ituri province after an infected person returned home, Congo’s health ministry said, complicating containment in a region beset by militia violence.

Ebola, which causes fever, vomiting and diarrhoea, finds a natural home in Congo’s vast equatorial forests. Continuing flare-ups have made the central African country a testing ground for new treatments against a virus that between 2013 and 2016 killed more than 11,300 people in a West African epidemic.

In an outbreak in western Congo which began in April and was declared over in July, an experimental vaccine manufactured by Merck & Co Inc (MRK.N) was given to 3,300 people and was considered central in containing the virus when it reached a city.

The mAb114 treatment was developed in the United States by the National Institutes of Health using the antibodies of the survivor of an Ebola outbreak in the western Congolese city of Kikwit in 1995.

World Health Organization Director-General Tedros Adhanom Ghebreyesus told a news conference in Geneva that medics were already treating five patients with mAb114 and that he had been informed they were doing well.

“We will use it as much as needed,” Tedros said. “But use of the molecules is decided by doctor and patient consent.”

Several other experimental treatments have arrived in the regional hub of Beni and are awaiting approval from an ethics committee, including Remdesivir (GILD.O), Favipiravir and REGN3450, REGN3471 and REGN3479, the health ministry said.

Separately, authorities have vaccinated more than 200 health workers and contacts of Ebola patients. He said the risk of international spread was currently considered low even though it poses a high regional risk because of its proximity to the Ugandan border, which is only about 100 km (60 miles) away.

The response is taking place against the backdrop of insecurity caused by dozens of militia groups who regularly kill and kidnap civilians in the region.

“Before I went there I was really worried because of the different nature of the Ebola outbreak in DRC,” Tedros said. “But after the visit I am actually more worried because of what we have observed there first hand.”

Authorities are reaching out to militia to convince them to allow access to zones they occupy, he said.