Ebola is threatening the very existence of Liberia, as the killer virus spreads like "wildfire", the country's defense minister warned on Tuesday, following a grim World Health Organization assessment that the worst is yet to come.

After predicting an "exponential increase" in infections across West Africa, the WHO warned that Liberia, which has accounted for half of all fatalities, could initially hope to slow the contagion but not stop it.

"Liberia is facing a serious threat to its national existence," Defense Minister Brownie Samukai told a meeting of the UN Security Council on Tuesday.

The disease is "now spreading like wildfire, devouring everything in its path", he said.

The WHO upped the Ebola death toll on Tuesday to 2,296 of the total 4,293 cases in Liberia, Sierra Leone, Guinea and Nigeria as of Sept 6. Nearly half of all infections had occurred in the past 21 days, it said.

The agency also evacuated its second infected medical expert, a doctor who had been working at an Ebola treatment center in Sierra Leone.

Emory University Hospital in the United States admitted a US national on Tuesday who had contracted the disease in western Africa, but declined to confirm whether the patient was the WHO employee.

The hospital has successfully treated two other infected US nationals.

Ebola, transmitted through bodily fluids, leads to hemorrhagic fever and - in more than half the cases - death. There is no specific treatment regime and no licensed vaccine.

The latest WHO figures underscore the asymmetric spread of Ebola, as it rips through densely populated communities with decrepit health facilities and poor public awareness campaigns.

Speaking on Tuesday, WHO's epidemiology chief Sylvie Briand said the goal in Senegal and Nigeria was now "to stop transmission completely". Senegal has announced only one infection, while Nigeria has recorded 19 infections and eight deaths.

The Democratic Republic of the Congo is battling a separate outbreak that has killed 32 in a remote northwestern region.

"But in other locations, like Monrovia, where we have really wide community transmission, we are aiming at two-step strategies - first, to reduce the transmission as much as possible and, when it becomes controllable, we will also try to stop it completely," Briand said in Geneva.

"But at this point in time, we need to be pragmatic and try to reduce it in the initial steps."

A day earlier, the WHO had warned that aid organizations trying to help Liberia respond would need to prepare to scale up their current efforts by three to four times.

Before the current outbreak, it noted, Liberia only had one doctor for every 100,000 patients in a population of 4.4 million.

In Montserrado county, which contains Monrovia, there are no spare beds at the few Ebola treatment sites operating, the WHO said.

AFP - Reuters