The virus spread undetected for weeks after the first known death as authorities tested for a more common type of Ebola and came up negative, health experts and aid workers said on Tuesday. The Bundibugyo virus has no approved medicines or vaccines.
Congo was expecting shipments from the United States and Britain of an experimental vaccine for different types of Ebola, developed by researchers at Oxford, said Jean-Jacques Muyembe, a virus expert at the National Institute of Biomedical Research.
"We will administer the vaccine and see who develops the disease," he said. But experts said such efforts would take time.
WHO Director-General Tedros Adhanom Ghebreyesus said he was "deeply concerned about the scale and speed of the epidemic", and pointed to the emergence of cases in urban areas, the deaths of healthcare workers and significant population movement.
In Congo, 30 cases have been confirmed, Tedros later told a meeting of the UN health agency's emergency committee.
Neighbouring Uganda has informed the WHO of two confirmed cases including a death in its capital, Kampala, among people who had travelled from Congo.
WHO has declared the Ebola outbreak a public health emergency of international concern, requiring a co-ordinated response.
Resources were being rushed to two affected provinces near Uganda. Parts of eastern Congo are in the hands of armed rebels, complicating sending aid.
The head of the WHO team in Congo, Dr Anne Ancia, said authorities hadn't identified "patient zero".
The Ervebo vaccine, used against a different type of Ebola, was among those considered for possible use, but anything approved would take two months to become available.
"I don't see that in two months we will be done with this outbreak," she said.
The UNICEF office in Bunia said it had been sent an initial 16 tonnes of relief supplies, mainly disinfectants and soaps, personal protective equipment and water purification tablets and water tanks.
Cases have been confirmed in the capital of Congo's Ituri province, Bunia; North Kivu's rebel-held capital, Goma; and the localities of Mongbwalu, Nyakunde and Butembo - home to well over a million people.
Tedros said an American had tested positive and been transferred to Germany.
Ebola is a highly contagious virus and can be contracted via bodily fluids such as vomit, blood or semen.
The disease it causes is rare but severe and often fatal. Symptoms include fever, headache, muscle pain, weakness, diarrhoea, vomiting, stomach pain and unexplained bleeding or bruising.
During an outbreak more than a decade ago that killed more than 11,000 people, many were infected while washing bodies for funerals.
There was growing panic in Bunia neighbourhoods as authorities urged people to remain calm and adhere to preventive measures including practising good hygiene and exercising caution during funerals.
The most important challenge is breaking the virus transmission chain, virus expert Muyembe said, adding that most of Congo's previous Ebola outbreaks "were brought under control simply by applying public health measures".
Congo has said the first person died from the virus on April 24 in Bunia, but the confirmation did not come for weeks. The body was repatriated to the Mongbwalu health zone, a mining area with a large population.
"That caused the Ebola outbreak to escalate," said Congo's health minister, Samuel Roger Kamba.
When another person fell ill on April 26, samples were sent to Congo's capital, Kinshasa, for testing.
On May 5, WHO was alerted of about 50 deaths in Mongbwalu, including four health workers. The first confirmation of Ebola came on May 14.
On the ground, the response is complicated by lack of resources.
