The World Health Organization said on Thursday that the number of Ebola cases was increasing rapidly and could exceed 20,000 before the virus is brought under control, as the death toll topped 1,500.
New figures showed the massive scale of the crisis, which the WHO said indicated a "rapid increase still in the intensity of transmission" that could cost at least $490 million (370 million euros) to tackle.
Bruce Aylward, the WHO's head of emergency programmes, said it could take six to nine months to bring Ebola 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," he told reporters in Geneva, calling the situation a "global health security issue".
"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.
Liberia was the worst affected with 694 deaths; 422 people have died in Sierra Leone; and 430 in Guinea, where the virus emerged at the start of the year. Nigeria has now recorded six deaths.
But Aylward warned that the actual caseload could be "two to four times higher than the number of cases you see reported".
Nigeria's latest death—in the southeastern oil city of Port Harcourt—was the first outside its biggest city, Lagos, and dashed hopes that the country had successfully contained the virus.
Scientists meanwhile said the first human trials of a potential vaccine will start next week using a product known as the NIAID/GSK Ebola vaccine candidate made by pharmaceuticals giant GlaxoSmithKline and the US government.
Health ministers from member states of the West African regional bloc ECOWAS met on Thursday in the Ghanaian capital Accra and vowed to strengthen their response to the devastating outbreak.
Ghana's President John Mahama, the current ECOWAS chairman, complained security measures taken by other countries to prevent the virus spreading, including travel bans, had unfairly hit member states.
"Currently in the sub-region, Ebola is officially reported in four countries and yet the entire West African sub-region of 15 nations and even Africa as a whole of 54 nations has been stigmatised," he said.
The bloc called for travel bans to be overturned and borders reopened.
The International Monetary Fund said it was studying additional financial support for Guinea, Sierra Leone and Liberia because of the "acute macroeconomic and social impact" on their already weak economies.
"Significant financing needs are likely to arise," it added.
In Nigeria, Health Minister Onyebuchi Chukwu said a doctor who treated a contact of the Liberian-American man who brought Ebola into the country died on August 22.
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.
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.
Shell and Total said they were both monitoring the situation closely and keeping staff informed of health advice. Operations have not yet been affected.
Emergency response hit
The director of the US Centers for Disease Control and Prevention, Tom Frieden, said on Wednesday "urgent action" was needed to tackle Ebola and warned the situation would get worse before it got better.
There has been mounting concern about the effect of the most lethal outbreak of the tropical virus in history.
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 also said they hoped safety trials on 140 healthy volunteers in Britain, Gambia and Mali could start next month and finish by the end of the year, with scope for GSK to make some 10,000 extra doses.
A number of foreign healthcare workers who contracted Ebola have been given an experimental therapy called ZMapp but stocks are exhausted.
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