The World Health Organization (WHO) said friday, Aug 22, the speed and extent of the Ebola outbreak in West Africa is "unprecedented".

The WHO's Dr Keiji Fukuda expressed concern over so-called "shadow zones", areas which cannot be reached and where patients are not being detected, BBC News reports.

The organisation confirmed 142 new cases of the disease had been reported since Aug 19, as well as 77 deaths. Already more people have died in this outbreak of Ebola than in any other. At least 1,427 people have now died in the outbreak so far, with the number of cases now standing at 2,615.

Speaking at a news conference in the Liberian capital Monrovia, Dr Fukuda said combating the disease would take "several months of hard work".

"We haven't seen an Ebola outbreak covering towns, rural areas so quickly and over such a wide area," he added.

Also on Friday, Nigerian authorities said that they had detected two new cases of the virus, both spouses of medical workers who had direct contact with a man who died from the virus in Nigeria after arriving from Liberia.

In Liberia on Friday, a boy of 16 shot during a protest about a quarantine died.

Shakie Kamara was one of three people seriously injured on Wednesday after security forces fired at protesters, angry after blockades were erected around the West Point slum.

Meanwhile, Senegal has defended the closure of its border with Guinea because of the Ebola outbreak, despite warnings that such measures are counterproductive.

The WHO says travel bans do not work, and that what is needed is more doctors and officials to help trace those infected with Ebola, and more mobile laboratories.

Senegal also banned flights and ships from Guinea, Liberia and Sierra Leone - the three worst-hit countries - however, Senegal's Health Minister Dr Eva Marie Colle Seck said humanitarian flights would not be affected.

Cameroon, Ivory Coast, Kenya and South Africa have also imposed travel bans.

Ebola has no known cure but some affected people have recovered after being given an experimental drug, ZMapp. However, supplies are now exhausted.

On Thursday, two U.S. doctors were discharged from hospital after being given the drug, while three Liberian medics are also recovering well.

The symptoms include high fever, bleeding and central nervous system damage.Fatality rate can reach 90% - but current outbreak has mortality rate of about 55%.Incubation period is two to 21 days.There is no 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.