Ebola-hit nations met for crisis talks on Thursday as the death toll topped 1,500 and the World Health Organization warned that the number of cases could exceed 20,000 before the outbreak is stemmed.

Nigeria announced that the virus had reached its oil-producing hub, dashing hopes that the country had successfully contained it to its biggest city, Lagos.

Hopes were raised meanwhile of a vaccine for the haemmorhagic fever after British medical charity the Wellcome Trust and pharmaceuticals giant GlaxoSmithKline said safety trials on a new drug could begin as soon as next month.

But new figures from the WHO showed the scale of the crisis. It said it was working on an assumption that it would take six to nine months to bring the epidemic under control, by which time the number of infections could have passed 20,000.

"That's not saying we expect 20,000, that's not saying we would accept, more importantly, 20,000 cases," Bruce Aylward, the WHO's head of emergency programmes, told reporters in Geneva.

"But we have got to have a system that is robust enough to deal with ... a very bad case scenario."

As of August 26, 1,552 people had been confirmed dead from Ebola in four countries -- Sierra Leone, Liberia, Guinea and Nigeria -- while 3,062 had been infected.

But Aylward warned that the actual caseload could be "two to four times higher than the number of cases you see reported."

- Economic threat -

Health ministers from member states of the West African regional bloc ECOWAS were meeting on Thursday in the Ghanaian capital Accra to discuss how to strengthen its response to the devastating outbreak.

As the meeting began, Nigeria's health minister Onyebuchi Chukwu said a doctor in the southeastern city of Port Harcourt had become the sixth person to die of Ebola and the first outside Lagos.

The medic died on August 22, a day after treating a patient who had contact with the Liberian-American man who first brought the virus to Nigeria and who died in a Lagos hospital on July 25.

The patient, an ECOWAS official, slipped through the net and travelled to Port Harcourt where he saw the doctor in a hotel room after feeling unwell, Chukwu said in Abuja.

News of the death came just a day after Nigerian officials claimed to have contained the spread of the virus.

Port Harcourt, 435 kilometres (270 miles) east of Lagos and the capital of Rivers state, is the centre of Nigeria's oil industry and a base for several international companies, including Anglo-Dutch giant Shell, US firm Chevron and France's Total.

A spokesman for Shell's Nigerian subsidiary said they were "monitoring the Ebola outbreak very closely and liaising with health authorities on the steps being taken to contain the disease".

Operations have not yet been affected. Nigeria is Africa's biggest oil producer, churning out roughly two million barrels a day, with crude accounting for more than 90 percent of its foreign exchange earnings.

- 'A tsunami at its peak' -

The director of the US Centers for Disease Control and Prevention, Tom Frieden, said on Wednesday "urgent action" was needed to address what the WHO has called an "unprecedented" outbreak.

Frieden told a news conference in the Liberian capital, Monrovia: "The cases are increasing. I wish I did not have to say this but it is going to get worse before it gets better."

There has been mounting concern about the effect of the most lethal outbreak of the tropical virus in history, which the WHO said could cost at least $490 million (370 million euros) to tackle over six months.

A number of airlines, including Air France and British Airways, have suspended their services to Freetown and Monrovia, leading to complaints that Ebola-hit countries were being increasingly isolated.

The WHO's Aylward said it was "absolutely vital" that airlines resume flights because it was hindering the emergency response. A travel ban was "a self-defeating strategy", he added.

On an Ebola vaccine, researchers said they hoped safety trials on 140 healthy volunteers in Oxford, the Gambia and Mali could finish by the end of the year, with scope for GSK to make some 10,000 extra doses.

Meanwhile, the World Food Programme likened the outbreak to "a tsunami that is already at its peak" after warnings of food shortages in affected countries.

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