WHO chief Tedros Adhanom Ghebreyesus, who declared the outbreak an international emergency last week, said he was “deeply concerned about the scale and speed of the epidemic”.
It is feared the outbreak may have been ongoing for several weeks before it was first detected on 24 April.
There is no vaccine for the strain of Ebola virus fuelling the latest rise in cases, but the WHO is evaluating whether other drugs may provide protection.
Speaking to BBC Newsday, Ancia said DR Congo’s Ituri province was a “very unsecured area with lots of movement of population”, making it difficult for the agency to investigate and help control the disease.
She continued: “The more we are investigating this outbreak, the more we realise that it has already disseminated at least a little bit across border and also in other provinces.”
The outbreak has spread to the province of South Kivu, where the population has been affected by a humanitarian crisis for many years, she added.
There has also been a case in eastern DR Congo’s biggest city, Goma, which has a population of around 850,000 people and is under the control of Rwandan-backed rebels.
High levels of insecurity in several provinces mean people move around often, increasing the risk and spread of the virus, she said.
Several African countries are taking precautions by tightening border screenings and preparing health facilities. Neighboring Rwanda has also closed its borders with the DR Congo. Uganda has told people to avoid hugging and shaking hands.
An American citizen, believed to be missionary group doctor Peter Stafford, is being evacuated from the DR Congo after developing symptoms over the weekend.
