WHO representative Dr Anne Ancia said the more investigations continue, the clearer it becomes that the virus has spread to additional provinces and crossed borders, raising concerns over the true scale of the epidemic, News.Az reports, citing BBC.

A modelling study released by the London-based MRC Centre for Global Infectious Disease Analysis suggested there may have been significant under-detection of cases and warned the actual number of infections could already exceed 1,000. Researchers said the outbreak is likely larger than currently confirmed and that its real magnitude remains uncertain.

WHO Director-General Tedros Adhanom Ghebreyesus, who declared the outbreak an international emergency last week, said he was deeply concerned about the speed and scale of the epidemic. Officials fear the virus may have been circulating for several weeks before it was first identified on April 24.

The outbreak is centred in Ituri province, an area described by WHO officials as highly insecure with constant population movement, making it difficult to investigate infections and contain transmission. Cases have also spread to South Kivu province and to the eastern city of Goma, which has a population of around 850,000 and is controlled by Rwandan-backed rebels.

Health agencies say there is currently no approved vaccine for the Bundibugyo strain responsible for the outbreak, although the WHO is evaluating whether existing drugs could provide some protection. The Red Cross warned that Ebola can escalate rapidly when cases are not identified early and healthcare systems are overwhelmed.

Several African countries have tightened border screening measures, while Rwanda has closed its border with DR Congo and Uganda has advised people to avoid physical contact such as handshakes and hugs. The United States is also evacuating several citizens exposed to the virus, including an American doctor who developed symptoms over the weekend.

Ebola initially causes flu-like symptoms including fever, headache and fatigue before progressing to vomiting, diarrhoea and, in severe cases, organ failure and bleeding. The virus spreads through contact with infected bodily fluids. The Bundibugyo strain is rare and has previously caused only two known outbreaks.