The death toll from the worst Ebola outbreak in history has jumped by almost 200 in a single day to at least 2,296 and is already likely to be higher than that, the World Health Organization said on Tuesday.

The WHO said it had recorded 4,293 cases in five West African countries as of Sept. 6, a day after its previous update. But it still did not have new figures for Liberia, the worst-affected country, suggesting the true toll is already much higher. The WHO has said it expects thousands of new cases in Liberia in the next three weeks.

'It's a war against this virus. It's a very difficult war.' - Sylvie Briand, WHO

As well as struggling to contain the disease, the organization is having difficulty compiling data on the number of cases, said Sylvie Briand, the director of WHO's department of pandemic and epidemic diseases.

"We know that the numbers are under-estimated," Briand told a news briefing in Geneva. "We are currently working to estimate the under-estimation.

"It's a war against this virus. It's a very difficult war. What we try now is to win some battles at least in some places."

The outbreak began last December and has been gathering pace for months, but about 60 per cent of Liberia's cases and deaths occurred within the last three weeks, the data showed.

More beds needed, MSF says

Medecins Sans Frontieres (MSF) said that Liberia's Montserrado County, which includes the capital, Monrovia, needs 1,000 beds to treat Ebola patients but the medical charity can only provide around 400 of those.

"We know that every day there are more people that need to be taken care of than we can include in our program. At the moment, there are insufficient beds," MSF emergency coordinator Laurence Sailly told a news conference on Tuesday.

Sailly said MSF was lobbying other non-governmental organizations and the United Nations to increase their response in the three countries, particularly in Liberia.

"We are working also in Guinea and Sierra Leone, so we will not be able to have more than 300 to 400 beds here in Montserrado. We are not going to go more than that, and it is not going to do anything with the scale of the epidemic here," Sailly said.

In Guinea and Sierra Leone, the other two countries at the centre of the outbreak, only 39 per cent of cases and around 29 per cent of deaths have occurred in the past three weeks, suggesting they are doing better at tackling the outbreak.

The new figures also showed two new suspected cases in Senegal in addition to one previously confirmed case there. In Nigeria, the overall number of cases fell to 21 from 22, as at least one suspected case turned out not be Ebola.

4th Ebola patient arrives in U.S.

Meanwhile, a fourth Ebola patient arrived in the United States from West Africa on Tuesday, headed to the same Atlanta hospital where two other people were successfully treated for the disease.

An air ambulance carrying the new patient landed Tuesday morning at Dobbins Air Reserve Base, Atlanta television station WSB-TV footage showed.

Emory University Hospital confirmed it would be treating the patient, who was not identified.

The hospital said in an earlier statement that the patient would be treated in the same isolation unit for serious infectious diseases where U.S. missionaries Nancy Writebol and Dr. Kent Brantly also received care for the lethal virus before being discharged last month.

Another American missionary, Dr. Rick Sacra, was also transported to the United States from West Africa after becoming infected with Ebola in Liberia. He is being treated at the Nebraska Medical Center in Omaha.

Medical workers have been hit hard by the epidemic, the worst since Ebola was discovered in 1976. As of late August, more than 240 healthcare workers had developed the disease and more than 120 had died, the WHO said.