Geneva - The Ebola epidemic is set to explode unless the response is radically intensified, the WHO said Tuesday, warning that hundreds of thousands could be infected by the end of the year.
The UN agency said in a report that new cases would surge from hundreds each week to thousands without "drastic improvements in control measures", with the number of infections set to more than triple to 20,000 by November.
"We've rather modestly only extended the projections to November 2, but if you go to... January 2, you're into hundreds of thousands," said Christopher Dye, the head of strategy at the World Health Organization and a co-author of the study.
The research paper warns that the outbreak could drag on for years and become entrenched in west Africa, which has already seen almost 3,000 deaths.
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The epidemic might simply "rumble on as it has for the last few months for the next few years", Dye said, adding that "the fear is that Ebola will become more or less a permanent feature of the human population".
Liberia, the hardest-hit nation, has seen 3,000 cases of Ebola and almost 1,600 deaths, with health workers turning people away from treatment units because of chronic shortages of beds and staff.
The country has some 150 foreign specialised medical workers on the ground but the United Nations has said they need at least 600, and health authorities are aiming to scale its current 400 Ebola beds up to around 2,000 within weeks.
Its response has been bolstered by a US military mission, already being deployed, which will see 3,000 troops providing training and logistics. Threat of civil war
But Antonio Vigilante, UN deputy special representative for recovery and governance in Liberia, likened the struggle to "trying to remedy an earthquake when it is happening".
Liberian Information Minister Lewis Brown has warned that the slow international response could result in a slide back into civil war or deadly unrest in the region.
"The effect of Ebola is being seen not just as a public health situation but it is also a political situation," he said, noting that Liberia has barely begun "rebuilding our capacity to live together (after two back-to-back civil wars from 1989-2003). This Ebola is threatening that capacity."
Sierra Leone, where more than 1 800 have been infected and nearly 600 have died, reported "an overflow of bodies" after a nationwide curfew helped uncover more than 200 new cases.
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The WHO study, carried out with Imperial College London and published in the New England Journal of Medicine, forecast the number of cases would rise to around 6 000 in Guinea, 10 000 in Liberia and 5 000 in Sierra Leone by November 2 without action.
And it warned that the fatality rate in the current outbreak was likely more than 70 percent rather than the current estimate of one in two, based on recovery rates rather than cases where the outcome was still unknown.
Meanwhile US health authorities unveiled their own worst-case scenario showing that, had international aid not begun to arrive, the virus would have been able to infect 1.4 million in Liberia and Sierra Leone by January.
Tom Frieden, the head of the US Centers for Disease Control and Prevention, said that while the model was based on out-of-date August data, it showed that "a surge now can break the back of the epidemic".
"It also shows that there are severe costs of delay," he told reporters.
The United Nations is seeking to raise nearly $1 billion (778 million euros) to defeat the Ebola outbreak, the worst ever recorded, which the Security Council has declared a threat to world peace.
Weak health systems
Ebola fever is one of the deadliest viruses known to man.
It can fell its victims within days, causing severe muscle pain, vomiting, diarrhoea and -- in many cases -- unstoppable internal and external bleeding.
The current crisis, which quietly began in southern Guinea last December, has killed far more than all other known Ebola outbreaks combined.
Dye said that while the virus ravaging west Africa was spreading similarly to previous outbreaks, what has changed is the density and mobility of the affected populations.
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Cultural practices like washing and touching dead bodies have compounded the problem, as has the slow response in affected countries, which have no previous experience with the virus, and on the part of the international community, he said.
Weak health systems in the hardest-hit countries are largely to blame, said Christl Donnelly, a professor of statistical epidemiology at the Imperial College and a co-author of the WHO study.
The first members of a team of 165 Cuban health workers arrived in Sierra Leone on Tuesday.
They will determine where the 62 doctors and 103 nurses will be deployed when they arrive early in October.
It will be the largest foreign medical team from a single country to offer to help deal with the outbreak.