People living close to the epicentre of a deadly Ebola outbreak have told the BBC of their fear, as the World Health Organization (WHO) warned cases may be spreading faster than originally thought.

One man in the Democratic Republic of Congo's north-eastern Ituri province said infected people were dying "very fast", adding: "Ebola has tortured us."

The virus is believed to have killed 136 people in the DR Congo, officials say, with more than 514 cases now suspected in the country. One person has died in neighbouring Uganda.

The WHO's Dr Anne Ancia told the BBC that the more the UN agency investigated the outbreak, the clearer it became that cases had spread to other areas.

Modelling by the London-based MRC Centre for Global Infectious Disease Analysis released on Monday suggested there had been "substantial" under-detection, and that it could not rule out there had already been more than 1,000 cases.

The study suggested that the current outbreak was "larger than currently ascertained" and that its "true magnitude remains uncertain".

A man who spoke to BBC News and identified himself as Bigboy said people were "really scared" and doing what they could to protect themselves.

He said locals were taking precautions such as washing hands with clean water, but added that he wished they could get access to other protective supplies such as face masks.

Another Ituri local, Alfred Giza, said people in the community were aware of the threat and waiting to receive face masks to protect themselves, but that he would not know what to do if a family member or friend contracted the disease.

The Red Cross warned that Ebola could escalate quickly if cases were not identified early, communities lacked information and health systems were overwhelmed, adding "we are seeing all those conditions" in the current outbreak.

On Tuesday, DR Congo President Félix Tshisekedi called for "calm" and urged Congolese citizens to remain vigilant, after holding a crisis meeting on Monday evening.

WHO chief Tedros Adhanom Ghebreyesus, who declared the outbreak an international emergency last week, said he was "deeply concerned about the scale and speed of the epidemic".

He declared the emergency late on Saturday night - before the WHO's emergency committee met to assess the situation and recommend priority medical interventions, which it is due to do soon.

The cash-strapped organisation has already released almost $4m (£3m) to combat the outbreak - but much more may be needed.

The outbreak is taking place in a region which has suffered years of conflict, with hospitals and clinics damaged or destroyed, and millions fleeing their homes. Most are living in unsanitary conditions.

There are also more than 11,000 refugees who have fled fighting in South Sudan, and big population movements among people seeking work in local gold mines.

It is feared the outbreak may have been ongoing for several weeks before it was first detected on 24 April.

There is no vaccine for the strain of Ebola virus fuelling the latest rise in cases, but the WHO is evaluating whether other drugs may provide protection.

Speaking to BBC Newsday, Ancia said DR Congo's Ituri province was a "very unsecured area with lots of movement of population", making it difficult for the agency to investigate and help control the disease.

She continued: "The more we are investigating this outbreak, the more we realise that it has already disseminated at least a little bit across border and also in other provinces."

WHO is sending tonnes of health supplies to DR Congo, including personal protective equipment (PPE) and medicine

The outbreak had spread to the province of South Kivu, where the population had been affected by a humanitarian crisis for many years, she added.

There has also been a case in eastern DR Congo's biggest city, Goma, which has a population of around 850,000 people and is under the control of Rwandan-backed rebels.

High levels of insecurity in several provinces mean people moved around often, increasing the risk and spread of the virus, she said.

Several African countries are taking precautions by tightening border screenings and preparing health facilities. Neighbouring Rwanda has also closed its borders with the DR Congo. Uganda has told people to avoid hugging and shaking hands.

An American citizen, believed to be missionary group doctor Peter Stafford, was evacuated from the DR Congo after developing symptoms over the weekend.

Germany's health ministry told the BBC a US citizen was taken to the country for treatment.

The US Centers for Disease Control and Prevention (CDC) said at least six other Americans who were exposed were also being evacuated. On Tuesday, the agency said it was in the process of moving the Americans to Germany and the Czech Republic, where they would be monitored for symptoms in quarantine.

The WHO and other agencies are working with governments and communities to try to stop the spread of the virus, urging residents to follow preventative measures and report to the nearest health facility if they experience any symptoms.

What is Ebola and how does it spread?

Ebola is caused by a virus and initially causes symptoms similar to the flu, with fever, headache and tiredness.

As the disease progresses, vomiting and diarrhoea develop and it can lead to organ failure. Some, but not all, patients develop internal and external bleeding.

The virus spreads from one person to another by contact with infected bodily fluids such as blood or vomit.

The Bundibugyo strain fuelling this rise in cases is rare, and has previously only caused two outbreaks, when it killed about a third of those infected.

Between 2014 and 2016, more than 28,600 people were infected by Ebola in West Africa, the largest outbreak of the virus since its discovery in 1976.

It was caused by the Zaire strain, for which there is an approved vaccine.

The disease spread to a number of countries in West Africa and beyond, including Guinea, Sierra Leone, the US, UK and Italy, killing 11,325 people.