There are many "blind spots" in the Ebola outbreak in the Democratic Republic of Congo, a World Health Organization (WHO) expert said on Friday, suggesting the spread of the deadly disease may be much wider than official estimates.

Congo said on Thursday the disease had spread ​to three new health ‌zones. It reported 676 confirmed cases and 136 deaths in an outbreak that has also spread to neighbouring Uganda.

"There are still many blind spots in some areas that are high risk," Olivier le Polain, a WHO epidemiologist in Beni, eastern Congo, said. 

"Surveillance really needs to be strengthened in those areas."

Another big challenge is a shortage of beds that medics can use to isolate patients, he said. There were only 250 across the three affected provinces, he added. 

The outbreak involves the rare Bundibugyo strain of Ebola, for which there is no approved treatment or vaccine.

The disease went undetected for weeks and first responders say they are playing catch-up. 

The WHO does not yet have projections for the size of the epidemic, Le Polain said, after the US CDC said it could be on the same level as the 2014-2016 West Africa outbreak which caused more than 11 000 deaths.