The Ebola epidemic is set to explode unless the response is radically intensified, says WHO, 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 out for years and become entrenched in west Africa, which has already seen almost 3,000 deaths.
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 due to chronic shortages of beds and staff.
The country has some 150 foreign specialised medical workers on the ground but the UN 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".
Liberia said Tuesday the slow international response risked allowing the country to slide back into civil war alongside neighbouring Sierra Leone, and could reignite civil unrest in Guinea.
"The world cannot wait for Liberia, Sierra Leone and Guinea to slip back into conflict, which could be the result of this slowness in response," Information Minister Lewis Brown told AFP late Monday.
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.
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.
"We are seeing exponential growth and we need to act now," Dye said.
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.
The UN has also produced a list of urgent "in kind" requirements, including helicopters, mobile laboratories, 3.3 million items of protective clothing and Ebola treatment centres.
- 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 speading similarly to previous outbreaks, what has changed is the density and mobility of the affected populations.
Cultural practices like washing and touching dead bodies have compounded the problem, as has the very slow response in affected countries, which have never before seen the virus, and the international community, he said.
Weak health systems in the hardest-hit countries are also largely to blame, said Christl Donnelly, a professor of statistical epidemiology at the Imperial College and a co-author of the study.
"In Nigeria, for example, where health systems are stronger, the number of cases has so far been limited, despite the introduction of infection into the large cities of Lagos and Port Harcourt," she said.
Ebola is only transmitted through contact with body fluids, so halting its spread is usually relatively simple.
Even in this epidemic, each Ebola patient on average infects only 1.7 people in Guinea, 1.8 in Liberia and two in Sierra Leone, the study showed.