"We are on the epidemiological knife edge," Peter Salama, in charge of emergency response at the World Health Organization (WHO), told a special meeting.
"The next few weeks will really tell if this outbreak is going to expand to urban areas or if we are going to be able to keep it under control," said Salama.
The agency issued a new toll, saying there had been 58 cases since the outbreak was declared on May 8, an increase of seven over figures issued on Tuesday, of which 27 have been deaths.
A team from Medecins Sans Frontieres dons protective clothing and equipment as they prepare to treat Ebola patients. Source: Medecins Sans Frontieres
One of the world's most notorious diseases, Ebola is a virus-caused haemorrhagic fever that in extreme cases leads to fatal bleeding from internal organs, the mouth, eyes or ears.
The outbreak began in rural northwestern DR Congo in a remote location called Bikoro.
Last Thursday, a first case was reported in Mbandaka -- a city of around 1.2 million people that lies on the Congo River, where it is a transport hub to Brazzaville and Kinshasa downstream and to Bangui, upstream.
So far, seven cases have surfaced in Mbandaka districts, WHO said.
"An urban case means that it can spread quickly. That is another challenge," WHO chief Tedros Adhanom Ghebreyesus told Wednesday's special session during the agency's annual World Health Assembly.
'Really serious'
The proximity of the outbreak to neighbouring countries, especially through the river connection, was a major concern, he said.
"They are connected, they are very close, and that is another challenge that makes the problem really serious," he said.
Salama said that another concerning factor was that five healthcare workers were among those infected.
"That is a tragedy in its own right, but it also signals the potential for further amplification," he said.
At the same time, though, the top WHO officials and DRC's ambassador to the UN in Geneva Zenon Mukongo Ngay, who spoke at the event, stressed the massive efforts put in place to halt the outbreak.
Salama pointed out that in just the two weeks since the outbreak was declared, clinical care facilities have been set up, an air bridge has been established to Bikoro, emergency financing has been mobilised, protective gear and emergency medical kits have been supplied.
In addition, a vaccination campaign has begun and more than 120 staff of WHO alone have been deployed alongside numerous staff from other organisations under the leadership of the DRC government.
