The number of Ebola infections will triple to 20,000 by November, soaring by thousands every week if efforts to stop the outbreak are not stepped up radically, the WHO warned in a study on Tuesday.

"If we don't stop the epidemic very soon, this is going to turn from a disaster into a catastrophe," said Christopher Dye, the head of strategy at the World Health Organization and a co-author of the study.

Unless reigned in, the outbreak could drag out for years and possibly become entrenched in West Africa, where it has already killed more than 2,800 people.

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On Tuesday, the U.S. Centers for Disease Control and Prevention is expected to release its own predictions for only Liberia and Sierra Leone that suggests cases could top a staggering 1 million by the end of the year. This with a mortality rate estimated now to be 71 percent.

The CDC calculations are based on assumptions that cases have been dramatically underreported. Other projections haven't made the same kind of attempt to quantify illnesses that may have been missed in official counts, USA Today reported.

CDC scientists conclude there may be as many as 21,000 reported and unreported cases in just those two countries as soon as the end of this month, according to a draft version of the report obtained by The Associated Press. They also predict that the two countries could have 550,000 to 1.4 million cases by late January.

"Without drastic improvements in control measures, the numbers of cases of and deaths from Ebola are expected to continue increasing from hundreds to thousands per week in the coming months," the study said.

"We've rather modestly only extended the projections to November 2, but if you go to ... January 2, you're into hundreds of thousands," Dye told reporters in Geneva.

The epidemic might simply "rumble on as it has for the last few months for the next few years," he said, stressing that "the fear is that Ebola will become more or less a permanent feature of the human population."

In the West African region reeling from the outbreak, the steepest challenge is currently facing Liberia, where more than 3,000 people have been infected, nearly 1,600 people have died, and where health workers turn away people begging for their lives from treatment units due to chronic shortages of beds and staff.

Sierra Leone, where more than 1,800 have been infected and nearly 600 have died, said Monday it had "an overflow of bodies", after a controversial nationwide lockdown helped uncover more than 200 new cases.

The WHO study, carried out with the Imperial College in London, forecast that if no significant action is taken, "the cumulative number of confirmed and probable cases by November 2 ... will be 5,925 in Guinea, 9,939 in Liberia and 5,063 in Sierra Leone".

The case total for those three countries alone will therefore surpass 20,000, said the study published in the New England Journal of Medicine on Tuesday.

Higher infection rates would translate into a jump in the number of deaths as the experts suggest the real fatality rate is much higher than the widely estimated one in two.

If only cases of deaths and recovery were taken into account, the fatality rate stands at about 71 percent, the study showed.

"We are seeing exponential growth and we need to act now," Dye said.

The United Nations is seeking to raise nearly $1.0 billion to defeat the worst-ever outbreak of Ebola, which the Security Council has declared a threat to world peace.

Ebola fever can fell its victims within days, causing severe muscle pain, vomiting, diarrhoea and -- in many cases -- unstoppable internal and external bleeding.

It is one of the deadliest viruses known to man, and the current crisis, which quietly began in southern Guinea last December, has by far killed more than all other Ebola outbreaks combined.

The previous deadliest outbreak was the very first one on record, in Democratic Republic of Congo in 1976, when 280 people died.

Dye said that while the virus ravaging West Africa was acting similarly to previous outbreaks, what has changed is the density and mobility of the population affected.

Cultural practices like washing and touching dead bodies have compounded the problem, as had the very slow response in countries never before hit by the virus as well as by 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 2.0 in Sierra Leone, the study showed.

"If it were an airborne transmitted disease ... we would be in much bigger trouble than we are at the moment," Dye said.

"It is possible," he said, that the disease could mutate to be more easily transmitted through the air, but "we certainly haven't see that yet."