Marburg virus disease is a viral hemorrhagic fever that can have a fatality rate of up to 88%, according to the WHO.
WHO chief Tedros Adhanom Ghebreyesus said there had been nine officially reported cases of the virus in Equatorial Guinea, with seven deaths.
These cases are in three provinces, some 150 kilometers apart, "suggesting wider transmission of the virus", he said.
"WHO is aware of additional cases and we have asked the government to report these cases officially to WHO."
On March 22, the WHO's Africa regional headquarters said it knew of 20 further probable cases, all of whom were dead.
The WHO's alert and response director Abdi Mahamud said there were "signs of the wide spread of transmission that are making us (worried)," adding: "This outbreak, as it stands, is larger and may be seen in more provinces."
"More than the case count number, it's the extent of the geographical spread."
WHO emergencies director Michael Ryan said countries had clear international obligations.
"When we're in the middle of an outbreak, and we have new and significant information, particularly related to lab-confirmed cases of dangerous pathogens, ... communities need to be made aware, put on the alert and able to take action," he said.
Any delay in releasing such information, Ryan said, especially when it relates to newly-affected areas, prevents that process.
"What we can't have is unnecessary delays in reporting disease," he said.
The WHO reported earlier that "On 7 February 2023, the Ministry of Health and Social Welfare of Equatorial Guinea reported at least eight deaths that occurred between 7 January and 7 February 2023, in two villages located in the district of Nsock Nsomo, eastern province of Kie-Ntem, Río Muni Region."
It also said "As of 21 February 2023, the cumulative number of cases is nine, including one confirmed case, four probable cases and four suspected cases. All the cases have died."
There is also an outbreak in Tanzania. WHO said it was working with the authorities and vaccine manufacturers to set up trials in the affected countries.
This report was sourced from Reuters and Agence France-Presse.