At least 131 people have died and more than 500 suspected Ebola cases have been reported in eastern Democratic Republic of the Congo, the country’s health ministry said Tuesday, as concerns grow over the government’s delayed response to the outbreak.

Samuel Roger Kamba, the country’s minister of public health, said authorities had recorded 513 suspected cases and 131 deaths across affected regions.

“These are suspected deaths, and investigations are underway to determine which ones are actually linked to the disease,” Kamba said.

The outbreak was officially declared a public health emergency of international concern on Sunday by the World Health Organization.

Health officials say the outbreak is linked to the Bundibugyo strain of the Ebola virus, a rare variant for which no approved vaccines or treatments currently exist.

Although Congo and neighboring Uganda have experienced more than 20 Ebola outbreaks over the years, this marks only the third known outbreak involving the Bundibugyo strain.

Cases have now been confirmed in several locations, including Bunia, Goma, Mongbwalu, Butembo and Nyakunde.

WHO Director-General Tedros Adhanom Ghebreyesus said Tuesday in Geneva that he was “deeply concerned about the scale and speed of the epidemic.”

Tedros said the WHO’s emergency committee would convene Tuesday to discuss additional recommendations aimed at containing the outbreak.