The World Health Organisation director-general openly worried Tuesday over the "scale and speed" of an outbreak of a rare Ebola variant in eastern Congo, where authorities reported a sharp increase in suspected deaths - to at least 131 - and over 500 suspected cases.

The virus spread undetected for weeks after the first known death as authorities tested for a more common strain and came up negative, health experts and aid workers said. This Bundibugyo virus, a rare variant, has no approved medicines or vaccines.

Congo's health minister, Samuel Roger Kamba, said investigations were underway to determine whether the deaths and 513 suspected cases were "actually linked to the disease."

WHO director-general Tedros Adhanom Ghebreyesus said he is "deeply concerned about the scale and speed of the epidemic," adding the UN health agency will convene its emergency committee later Tuesday. He pointed to the emergence of cases in urban areas, the deaths of healthcare workers and significant population movement.

The WHO says patient zero has not yet been confirmed

The WHO has declared the Ebola outbreak a public health emergency of international concern, requiring a coordinated response. Resources were being rushed to the two affected provinces near the border with Uganda, which has reported one death in a person who travelled from Congo.

The head of the WHO team in Congo said authorities haven't identified "patient zero" in the outbreak.

Dr Anne Ancia also said the Erbevo vaccine, used against a different Ebola strain, was among those being considered for possible use. But even if that or another is approved, it would take two months to become available.

Inside Congo, cases have been confirmed in the capital of Ituri province, Bunia; North Kivu's rebel-held capital, Goma; and the localities of Mongbwalu, Nyakunde and Butembo - home to well over a million people in all.

Dr Peter Stafford, an American doctor, is among the Bunia cases, said the Christian organisation he works for, Serge. He had been treating patients at a hospital. Three other Serge employees were working there, including Stafford's wife, but were not showing symptoms.

False negative Ebola tests delayed the response

Congo has said the first person died from the virus on April 24 in Bunia, and the body was repatriated to the Mongbwalu health zone, a mining area with a large population.

"That caused the Ebola outbreak to escalate," said Kamba, the health minister.

When another person fell ill on April 26, samples were sent to Congo's capital, Kinshasa, for testing, according to the Africa Centres for Disease Control. Bunia is more than 1,000 kilometres (620 miles) away in a country with some of the world's worst infrastructure.

Samples from Bunia were initially tested for the more common type of Ebola, Zaire, Congolese officials said. They came back negative, said Dr Richard Kitenge, the health ministry incident manager for Ebola, and local authorities assumed it was not the virus.

Only laboratories in Kinshasa and Goma, which are now controlled by the M23 rebel group, have the capacity to test for the Bundibugyo virus. It was not clear what measures the Rwanda-backed rebels were taking in the outbreak.

On May 5, the WHO was alerted to about 50 deaths in Mongbwalu, including four health workers. The first confirmation of Ebola came on May 14.

"Our surveillance system didn't work," said Jean-Jaques Muyembe, a virologist at the National Institute of Bio-Medical Research.

"The Bunia laboratory ... should have continued searching and sent the samples to the national laboratory. Something went wrong there. That's why we ended up in this catastrophic situation," he said, and asserted that members of parliament and senators were aware "there were deaths, and nothing was being said."

Matthew M. Kavanagh, director of the Georgetown University Centre for Global Health Policy and Politics, criticised the Trump administration's earlier decision to withdraw from the WHO and make deep cuts in foreign aid - "the exact surveillance system meant to catch these viruses early," he said.

The US State Department pushed aside criticism on Monday, saying it sprang into action immediately and has provided USD 13 million in assistance for the response.

This is a rare type of Ebola

Ebola is highly contagious and can be contracted via bodily fluids such as vomit, blood or semen. The disease it causes is rare but severe and often fatal. During an outbreak over a decade ago that killed over 11,000, many were infected while washing bodies during community funerals.

"Ebola is very much a disease of compassion in that it impacts the people who are more likely to be taking care of sick folks," said Dr Craig Spencer, an associate professor at the Brown University School of Public Health who survived Ebola more than a decade ago after contracting it in Guinea.

The US CDC says it causes fever, headache, muscle pain, weakness, diarrhoea, vomiting, stomach pain and unexplained bleeding or bruising.

The severity of the symptoms and the rising caseload were fueling growing panic in Bunia neighbourhoods.

"I know the consequences of Ebola, I know what it's like," said resident Noëla Lumo. She previously lived in Beni, a region hit by former outbreaks. When she heard about the latest one, she began making protective masks by hand.

The region already grapples with a humanitarian crisis

Eastern Congo has long grappled with a humanitarian crisis and the threat of armed groups that have killed dozens and displaced thousands in Ituri in the past year. Ituri already had over 273,000 displaced people out of a population of 1.9 million, according to the UN

UN staff have been asked to work from home and avoid physical contact and crowded areas, said a Bunia-based UN official, who spoke on condition of anonymity because they were not authorised to speak publicly on the subject.

The most important challenge is breaking the virus transmission chain, Muyembe said.

"Of the 17 epidemics we have experienced in (Congo), 15 were brought under control simply by applying public health measures," he said. "The disease is transmitted through contact with bodily fluids. If you avoid this contact, you break the chain of transmission, and the epidemic stops."

Vaccine options

A panel of experts led by the World Health Organisation meets on Tuesday to discuss whether there are any vaccine options to help tackle an Ebola outbreak in eastern Democratic Republic of Congo.

The virtual meeting follows more than 130 suspected deaths and 500 cases linked to the outbreak of the Bundibugyo strain of Ebola, according to the WHO, which, along with the Africa Centers for Disease Control and Prevention, has declared it a public health emergency.

There are no approved vaccines or treatments for the Bundibugyo strain of Ebola, which has a fatality rate of up to 40%.

However, there is a vaccine named Ervebo, manufactured by Merck, that is used for the Ebola Zaire strain but has shown evidence of providing some protection against Bundibugyo in animal studies.

The potential for testing this and other options will be on the agenda of the meeting of the WHO Technical Advisory Group, the WHO and other scientists said. But the final decision will lie with the governments of Congo and neighbouring Uganda, where two confirmed cases have also been found. Potential treatments will also be discussed.

"When you have an outbreak with a strain that does not have countermeasures, we are going to advise on the best approach to take," said Dr Mosoka Fallah, acting director of the science department at Africa CDC. "We will look at what evidence we have and make a decision."

The vaccine group Gavi holds a stockpile of Ebola vaccines. Its chief executive, Sania Nishtar, said 2,000 doses were already in Congo, if the experts decided it should be used in a trial there.

Some experts said the outbreak could still be difficult to control.

"Our concern about this outbreak is pretty high," said Richard Hatchett, head of the Coalition for Epidemic Preparedness Innovations, which funds the development of new vaccines and is looking at potential candidates.