Marburg virus disease is a viral haemorrhagic fever that can have a fatality rate of up to 88 per cent, according to the WHO.
Symptoms include fever, fatigue and blood-stained vomit and diarrhoea.
There are no vaccines or antiviral treatments approved to treat it.
Marburg is passed on to people from fruit bats and is from the same virus family responsible for the deadly Ebola disease.
Nine people have died while one patient has recovered since the beginning of the epidemic, Equatorial Guinea's health ministry said on Twitter, adding that 825 contacts have since been traced.
The country confirmed its first-ever outbreak of the disease in February, according to WHO, which last week had reported nine laboratory-confirmed cases and put the total number of deaths and probable cases at 20 each.
"WHO is aware of additional cases and we have asked the government to report these cases officially to WHO," its director general Tedros Adhanom Ghebreyesus said earlier on Wednesday.
There is also an outbreak of Marburg virus in Tanzania, where eight cases including five deaths have been reported in the northwest Kagera region, the WHO has said.
The WHO said it was working with local authorities and vaccine manufacturers to set up trials in the affected countries.
