A leading charity has warned that a rate of five new Ebola cases an hour in Sierra Leone means healthcare demands are far outstripping supply.
Save the Children said there were 765 new cases of Ebola reported in the West African state last week, while there are only 327 beds in the country.
Experts and politicians are set to meet in London to debate a global response to the crisis.
It is the world's worst outbreak of the virus, killing 3,338 people so far.
There have been 7,178 confirmed cases, with Sierra Leone, Liberia and Guinea suffering the most.
Save the Children says Ebola is spreading across Sierra Leone at a "terrifying rate", with the number of new cases being recorded doubling every few weeks.
It said that even as health authorities got on top of the outbreak in one area, it spread to another.
The scale of the disease is also "massively unreported" according to the charity, because "untold numbers of children are dying anonymously at home or in the streets".
"We're in a race against time," said Justin Forsyth, the organisation's chief executive.
Speaking on the BBC's Today programme he said that the figure for Sierra Leone could rise to 10 people every hour before the end of the month if urgent action were not taken.
Meanwhile the head of the UN's mission to combat Ebola warned that the disease was spreading "very rapidly" and that a "massive international response" was required to deal with the crisis.
Anthony Banbury, who is in Liberia, said more needed to be done to educate remote communities about how to protect themselves from infection.
"Cases are doubling every 20 days," he said. "The disease has now reached every county in Liberia."
Earlier this month, Britain said it would build facilities for 700 new beds in Sierra Leone but the first of these will not be ready for weeks, and the rest may take months.
Safety trials for two experimental vaccines are under way in the UK and US, the WHO said on Wednesday, and will be expanded to 10 sites in Africa, Europe and North America in the coming weeks.
It said it expected to begin small-scale use of the experimental vaccines in West Africa early next year.
The Ebola Donors Conference in London on Thursday is being hosted by the UK and Sierra Leone governments. Its main agenda is to discuss what the global community can do to provide an effective international response.
Ministers, diplomats and health chiefs were due to arrive from about 20 different countries, including the US, France, Japan, Australia and all of the West African nations hit by the disease.
UK Foreign Secretary Philip Hammond asked the international community to back an emergency pilot scheme to stop the spread of Ebola in Sierra Leone.
He said that the initiative involved setting up community health care centres to isolate patients at the early stages of infection when Ebola is less contagious.
"The alternative is allowing this disease to progress," said Mr Hammond, citing a worst-case scenario issued by experts that one and a half million people could be infected by January.
The conference was also attended by William Pooley, the British nurse who contracted Ebola in Sierra Leone but has since made a full recovery.
He said he did not know what would happen if the "horror and the misery" he witnessed was repeated a million times.
The president of Sierra Leone, Ernest Bai Koroma, cancelled his attendance at the last minute.
The UK Foreign Office said this was due to his presidential jet breaking down before take-off.
An FCO spokesman said that delegates were hoping Mr Koroma would instead participate via video link.
The event has been billed as a "pledging conference" designed to raise millions of pounds for a UN fighting fund.
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Spread by body fluids, such as blood and saliva
- Fatality rate can reach 90% - but current outbreak has mortality rate of about 70%
- Incubation period is two to 21 days
- There is no proven vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats, a delicacy for some West Africans, are considered to be virus's natural host