Democratic Republic of Congo faces a "very high" public health risk from Ebola because the disease has been confirmed in a patient in a big city, the World Health Organization (WHO) said on Friday, raising its assessment from "high" previously.
The risk to countries in the region was now "high", raised from "moderate", but the global risk remained "low", the WHO said.
The reassessment came after the first confirmed case in Mbandaka, a city of around 1.5 million on the banks of the Congo River in the northwest of the country.
The case raised concerns that the virus, previously found in more rural areas, would be tougher to contain and could reach downstream to the capital Kinshasa, which has a population of 10 million.
It also followed the announcement by Congo's health ministry of 11 newly confirmed cases in the smaller town of Bikoro, near the northwest village where the virus was first detected.
"The confirmed case in Mbandaka, a large urban centre located on major national and international river, road and domestic air routes, increases the risk of spread within the Democratic Republic of the Congo and to neighbouring countries," the WHO said.
WHO Deputy Director-General for Emergency Preparedness and Response Peter Salama had told reporters on Thursday that the risk assessment was being reviewed.
"Urban Ebola is a very different phenomenon to rural Ebola because we know that people in urban areas can have far more contacts so that means that urban Ebola can result in an exponential increase in cases in a way that rural Ebola struggles to do."
Later on Friday, the WHO will convene an Emergency Committee of experts to advise on the international response to the outbreak, and decide whether it constitutes a "public health emergency of international concern".
The nightmare scenario is an outbreak in Kinshasa, a crowded city where millions live in unsanitary slums not connected to a sewer system.
Jeremy Farrar, an infectious disease expert and director of the Wellcome Trust global health charity, said the outbreak had "all the features of something that could turn really nasty".
"As more evidence comes in of the separation of cases in space and time, and healthcare workers getting infected, and people attending funerals and then travelling quite big distances - it's got everything we would worry about," he told Reuters.
WHO spokesman Tarik Jasarevic said on Friday that Congo's Ministry of Health had provided updated figures: 45 cases overall since April 4, including 14 confirmed, 10 suspected and 21 probable.
There had been 25 deaths, but no new infections among healthworkers, Jasarevic told reporters.
The WHO is sending 7,540 doses of an experimental vaccine to try to stop the outbreak in its tracks, and 4,300 doses have already arrived in Kinshasa. It will be used to protect health workers and "rings" of contacts around each case.
The vaccine supplies will be enough to vaccinate 50 rings of 150 people, the WHO said. Each ring represents the number of people including health workers who may have come into contact with an Ebola patient.
As of 15 May, 527 contacts had been identified and were being followed up and monitored.