An outbreak of the deadly Ebola virus in Democratic Republic of Congo (DRC) has the clear "potential to expand" as the number of confirmed cases continues to rise, the World Health Organization (WHO) warned.

Health workers' response is on an "epidemiological knife's edge" after the number of people stricken with Ebola in the DRC rose to 28 since an outbreak was detected earlier this month, said WHO Deputy Director Peter Salama, in comments made on Wednesday at a conference in Geneva, Switzerland.

Seven of the confirmed cases were in urban settings .

"The next few weeks will really tell if this outbreak is going to expand to urban areas or if we're going to be able to keep it under control," Salama said.

The average fatality rate among those infected with Ebola, which has no proven cure, is about 50 percent, according to WHO.

DRC's most recent Ebola outbreak - its ninth since the disease was first identified in 1976 - initially appeared to be confined in a rural setting near the town of Bikoro, in the central African nation's northwestern Equateur Province.

But a confirmed instance of the virus last week in the city of Mbandaka, home to 1.2-million people and about 150km away from Bikoro, plunged the ongoing crisis into a "new phase", the DRC's Health Minister Oly Ilunga said last week.

Twenty-seven people have died and at least 58 others in DRC's northwest have shown Ebola symptoms since it was identified on May 8 , according to the health ministry.

Regional response

On Wednesday, WHO said it would work with nine countries neighbouring DRC in a bid to prevent the virus spreading across borders.

Matshidiso Moeti, WHO director for Africa, told the conference Central African Republic and Republic of Congo were the organisation's top priority countries because of their proximity.

Efforts to detect and stem a possible cross-border spread would also be rolled out in Angola, Burundi, Rwanda, South Sudan, Tanzania, Zambia and Uganda, Moeti said.

The WHO has sent 7 540 experimental vaccines to DRC so far. It will send another 8 000 doses, made by pharmaceutical firm Merck, over the next few days.

Though unlicensed, the experimental vaccine proved effective when used in trials in West Africa between 2013-2016 during an Ebola outbreak, which killed about 11 300 people as it surged through Guinea, Sierra Leone, and Liberia.

Ebola, which can cause multiple organ failure, is passed from human to human by contact through the mouth, nose, or broken skin with blood or other bodily fluids of those infected.