Ebola is spreading exponentially in Liberia, with thousands of new cases expected in the next three weeks, the World Health Organization (WHO) says.
Conventional methods to control the outbreak were "not having an adequate impact", the UN's health agency added.
At least 2,100 people infected with Ebola have died so far in the West African states of Guinea, Liberia, Sierra Leone and Nigeria this year.
The WHO says 79 health workers have been killed by the virus.
Organisations combating the outbreak needed to scale-up efforts "three-to-four fold", the WHO said.
It highlighted Liberia's Montserrado county, where 1,000 beds were needed for infected Ebola patients but only 240 were available, leading to people being turned away from treatment centres.
Transmission of the virus in Liberia was "already intense", and taxis being used to transport infected patients appeared to be "a hot source of potential virus transmission", the WHO said.
"As soon as a new Ebola treatment facility is opened, it immediately fills to overflowing with patients, pointing to a large but previously invisible caseload," it added.
"When patients are turned away... they have no choice but to return to their communities and homes, where they inevitably infect others."
The international response to the crisis has been stepped up, with the UK and US both promising to open new treatment centres in West Africa.
The British military said it would build a 50-bed centre near Freetown, the capital of Sierra Leone, while the US announced that it would send a 25-bed field hospital to Liberia at a cost of $22 million.
Analysis: James Gallagher, BBC health editor
Three countries - Guinea, Sierra Leone and Liberia - are at the heart of the Ebola outbreak, but Liberia is suffering the most by far. Why this is the case is not completely understood. Finding the answer will be a critical part of tackling the outbreak.
Variations in burial practice - which can include touching the body and eating a meal near it - are being investigated.
There are also questions about trust in the authorities and how the risk of Ebola is being communicated. Riots erupted in the West Point slum, with some reports suggesting protesters believed Ebola was a hoax.
Another aspect is the state of the healthcare system, which was left in ruin by the civil war. Liberia had one doctor per 100,000 people before Ebola killed several staff.
The response has also been lacking. In the capital Monrovia there are 240 beds, but experts say they need more than 1,000. Patients without a bed have no choice but to go back home, where they may spread the virus.
The Ebola disease spreads between humans by direct contact with infected blood, bodily fluids or organs, or indirectly through contact with contaminated environments.
Conventional means of controlling the outbreak, which include avoiding close physical contact with those infected and wearing personal protective equipment, were not working well in Liberia, the WHO said.
However, they appeared to be more effective in "areas of limited transmission" such as Nigeria and Senegal, it added.
Local communities, especially those in rural areas, had been able to slow the transmission when they put in place their own protective measures, the WHO statement said.
Also on Monday, the African Union urged its member states to lift travel bans imposed to contain the virus, saying that the bans could hurt the region's economy.
"We must be careful not to introduce measures that may have more... social and economic impact than the disease itself," commission chief Nkosazana Dlamini-Zuma said in quotes carried by AFP news agency.
The current outbreak has mortality rate of about 55%.
Liberia has the highest number of reported cases and deaths, with more than 1,000 casualties so far.
Hundreds have also died of the virus in Guinea and Sierra Leone.
There have been at least eight deaths in Nigeria. One case has also been confirmed in Senegal but there have been no deaths so far.