The Ebola outbreak in the Democratic Republic of the Congo has spread to a second major city and 13 more people have died in three days.
Butembo, a city of one million people 35 miles (56km) away from the city of Beni where most of the outbreak has been raging, is now reporting cases of the deadly fever.
Experts warn the quick spread makes tackling the virus more complicated because containing it has been challenging enough in the one city.
The outbreak, which has killed 273 people already, began in August and has become the second worst Ebola outbreak in history.
Experts now fear experimental vaccines which have been doled out to thousands of people, and have reportedly prevented the death toll rising into the thousands, will run out.
So far 18 people in Butembo have had an Ebola diagnosis confirmed and all 18 of them have died.
This is a small proportion of the total 471 suspected cases of the fast-spreading virus, but it is difficult to contain and could quickly spiral out of control in the city.
'We are very concerned by the epidemiological situation in the Butembo area,' said John Johnson, project coordinator with Doctors Without Borders in the city.
New cases are increasing quickly in the eastern suburbs and outlying, isolated districts, the medical charity said.
The outbreak declared on August 1 is now second only to the devastating West Africa outbreak that killed more than 11,300 people a few years ago.
Without the teams vaccinating more than 41,000 people already, this outbreak could have already seen more than 10,000 Ebola cases, the health ministry said.
This is by far the largest deployment of the promising but still experimental Ebola vaccine, which is the first drug of its kind to be publicly rolled out.
The manufacturer, Merck, keeps a stockpile of 300,000 doses and preparing them takes months.
'We are extremely concerned about the size of the vaccine stockpile,' WHO's emergencies director, Dr Peter Salama, told the STAT media outlet.
He said 300,000 doses will not be enough as urban Ebola outbreaks become more common.
Health workers, contacts of Ebola victims and their contacts have received the vaccine in a 'ring' approach around victims.
And in some cases, all residents of hard-to-reach communities have been offered it.
The prospect of a mass vaccination in a major city like Butembo has raised concerns and Dr Salama called the approach 'extremely impractical.'
A WHO spokesman said shipments of doses arrive almost every week to ensure a sufficient supply for the ring vaccination.
'No interruptions of vaccine supply have occurred to date,' the WHO's Tarik Jasarevic told Associated Press.
'Merck is actively working to ensure sufficient number of doses continue to be available to meet the potential demand.'
This Ebola outbreak is like no other, with deadly attacks by rebel groups forcing containment work to pause for days at a time.
Yesterday in Beni, 18 civilians were killed in two separate attacks by militants, who kidnapped groups of people, dragged them out of the city into the suburbs, burned down a house and executed some of the hostages.
Some locals are wary of health workers and resist vaccinations or safe burials of Ebola victims because people there have never encountered the virus before.
A 'fringe population' has regularly destroyed medical equipment and attacked workers, Health Minister Dr Oly Ilunga Kalenga told reporters on Wednesday.
The Ebola virus is spread via bodily fluids of those infected, including the dead.
The outbreak 'remains serious and unpredictable,' the World Health Organization said in an assessment released Wednesday.
Nine health zones have reported new cases in the last week, and some have been unrelated to known victims, meaning there are still gaps in tracking the virus.
This is an issue because the North Kivu region, where the outbreak is raging, has dense, highly mobile population.
Thousands of people have been organised by Red Cross societies and others to go house-to-house dispelling rumors and checking on possible contacts of victims.
Dr Fatoumata Nafo-Traore, Africa's regional director for the International Federation of Red Cross and Red Crescent Societies, joined one awareness campaign in the outbreak's epicenter, Beni, this week.
The head of one family thanked her for the face-to-face contact, saying he didn't even have a radio and didn't understand what was happening.
'Ignorance is the enemy,' another resident said.
Given the years of conflict in eastern Congo, it's essential for households to understand and trust why the health workers are there, Dr Nafo-Traore said.
While she called the insecurity 'very worrying,' she said that with new tools at hand, including vaccines, 'there is great hope.'
Another obstacle facing health workers in the region recently was a spike in malaria cases which are driving people to health centres where they're catching Ebola.
Half of people suspected of having Ebola in medical centres in Beni actually just had malaria, health workers said recently.
'It will make things a lot easier if malaria is taken out of the equation,' said Stefan Hoyer of the World Health Organization (WHO).
Workers in the city of Beni launched a four-day door-to-door blitz last week to try and stem the flow of malaria cases.
They gave out mosquito nets and anti-malarial drugs to 450,000 people to stop them going to medical centres where they may catch Ebola.
'We can assume that the suspected Ebola cases to be triaged would at least go down by half,' Mr Hoyer added.
This is what happened in Sierra Leone's capital, Freetown, when people with malaria were filling Ebola treatment centres during the West African outbreak in 2014, he said.
The treatment of Ebola itself has taken an experimental turn in DRC, where scientists are now conducting a real-time study of how well pioneering drugs work.
More than 160 people there have already been treated with the drugs, and the way people are treated won't change, but scientists will now be able to compare them.
The announcement came after two devastating weeks in which the death rate has been high and officials confirmed even newborn babies are catching the virus.
Four experimental drugs are being used to try and combat the disease – mAb 114, ZMapp, Remdesivir and Regeneron.
Patients will get one of the four, but researchers won't know which they were given until after the study.
By comparing how well these work, scientists will be moving towards curing the disease and slashing the death tolls in future outbreaks.
'While our focus remains on bringing this outbreak to an end, the launch of the randomised control trial is an important step toward finally finding an Ebola treatment that will save lives,' said WHO director-general Dr Tedros Adhanom Ghebreyesus.
Because the data collected in the North Kivu epidemic is unlikely to be sufficient for a complete study, the country's health ministry said the clinical trial may extend over a five-year period to cover Ebola outbreaks in other countries.
'Our country is struck with Ebola outbreaks too often, which also means we have unique expertise in combatting it,' said DRC's minister of health Dr Oly Ilunga Kalenga.
'These trials will contribute to building that knowledge, while we continue to respond on every front to bring the current outbreak to an end.'
The outbreak has been plagued by security problems, with health workers attacked by rebels in districts where the virus has been spreading.