Despite "encouraging signs" in the fight against the Ebola outbreak in the Democratic Republic of Congo, the World Health Organization has warned that efforts to eliminate the virus are "far from over."

WHO chief Tedros Adhanom Ghebreyesus told journalists at an online news conference that cases were surging in North Kivu province as officials raced to control the spread of "many outbreaks in many places."

The number of weekly cases in North Kivu had doubled over the past two weeks, rising from 100 to more than 200, Tedros said.

"The area is so vast that it's hard to speak of a single epidemic," he added.

Tedros noted that transmission was declining in the worst-affected areas of Ituri province, the epicentre of the outbreak, while South Kivu had not reported any new cases since May.

"But make no mistake, the epidemic continues to grow and continues to kill," Tedros warned.

The outbreak has been further complicated by the emergence of a rare strain of Ebola known as the Bundibugyo virus, for which there is no known cure.

WHO said treatment trials and the development of post-exposure prophylaxis were accelerating. More than 3,000 healthcare and front-line workers had already been vaccinated with Merck's Ervebo vaccine.

Ervebo is licensed against the Zaire ebolavirus, but its effectiveness against the Bundibugyo strain, which is driving the current outbreak, is not known, according to WHO's director of science, research, evidence and quality for health, Meg Doherty.

More than 7,200 cases and more than 3,500 deaths have been reported across seven provinces since the DRC declared its 17th Ebola outbreak in May.