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 northeastern 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 investigates the outbreak, the clearer it becomes cases have 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 is 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 are really scared and doing what they can to protect themselves. He stated locals are 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 are 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 can escalate quickly if cases are not identified early, communities lack information and health systems are 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.

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.

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, with plans to monitor them 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.