Liberia's chief medical officer has put herself and her staff in isolation after her aide died of Ebola, officials have said.

Dr Bernice Dahn, who is also a deputy health minister, opted to put herself in quarantine on Sunday following her assistant's death on Thursday.

Dahn and her assistant's staff, whom she also quarantined, will remain under observation for 21 days, the full incubation period of the virus.

Four West African nations had confirmed cases of Ebola.

Liberia has been hardest hit. According to a World Health Organisation count released on Saturday, 1,830 or 3,458 people infected in the country had died. 

Healthcare workers, already in very short supply, have paid an especially heavy price, with 211 dead in the stricken nations, 89 of them in Liberia.

A WHO official told the AFP news agency that the organisation estimated between 35 to 40 died of the virus each day in the Liberian capital, Monrovia.

"This is a slow rise, and it only includes the cases that have been officially tallied. There are still people who continue to bury their dead secretly in their gardens," he said.

The US federal health body Centres for Disease Control and Prevention estimates that only about 40 percent of Ebola cases are being announced in Liberia and Sierra Leone, the worst-affected states.

The virus can kill its victims within days, causing rampant fever, severe muscle pain, vomiting, diarrhea and in many cases unstoppable internal  and external bleeding.

Liberia's decrepit public health infrastructure, ruined by 14 years of civil war to 2003 and endemic poverty, has "totally collapsed" under the Ebola crisis, the WHO official said.

Source: Agencies