The WHO said the outbreak, caused by the Bundibugyo virus, does not meet the criteria of a pandemic emergency but that countries sharing land borders with the DRC are at high risk of further spread.
On Sunday, the United Nations health agency said 80 suspected deaths, eight laboratory-confirmed cases and 246 suspected cases had been reported as of Saturday in the DRC's Ituri province across at least three health zones, including Bunia, Rwampara and Mongbwalu.
One case was confirmed in the eastern Congolese city of Goma, a statement by M23 rebels said.
The DRC health ministry had said on Friday that 80 people had died in the new outbreak in the eastern province.
The 17th outbreak in the country, where Ebola was first identified in 1976, could be much larger, given the high positivity rate of the initial samples and the increasing number of suspected cases being reported, the WHO said.
The outbreak is "extraordinary" as there are no approved Bundibugyo virus-specific therapeutics or vaccines, unlike for Ebola-Zaire strains, it said. All but one of the country's previous outbreaks were caused by the Zaire strain.
The DRC-Uganda outbreak poses a public health risk to other countries, with some cases of an international spread already documented, the agency said, advising countries to activate their national disaster and emergency-management mechanisms and undertake cross-border screening and screening at main internal roads.
Officials from the US Centers for Disease Control and Prevention told reporters on Sunday the agency has activated its emergency response centre for the outbreak and plans to send more people to its offices in the DRC and Uganda. Satish Pillai, CDC Ebola response incident manager, declined to say whether any Americans were among those who had been infected.
In Uganda's capital, Kampala, two apparently unrelated laboratory-confirmed cases, including one death, were reported on Friday and Saturday, from people travelling from the DRC, the WHO said.
The WHO on Sunday said a previously reported laboratory-confirmed case in Kinshasa, the capital of the Democratic Republic of Congo, had tested negative after secondary testing was done.
Bundibugyo virus-disease contacts or cases should not travel internationally, unless as part of a medical evacuation, the WHO said.
The agency advised immediately isolating confirmed cases and monitoring contacts daily, with restricted national travel and no international travel until 21 days after exposure.
At the same time, the WHO urged countries not to close their borders or restrict travel and trade out of fear, as this could lead to people and goods making informal border crossings that are not monitored.
The DRC's dense tropical forests are a natural reservoir for the Ebola virus.
The often-fatal virus, which causes fever, body aches, vomiting and diarrhoea, spreads through direct contact with the bodily fluids of infected people, contaminated materials or persons who have died from the disease, according to the Africa Centres for Disease Control and Prevention.
