The World Health Organisation is warning that West Africa's Ebola outbreak, which has killed more than 700 people since February, is spreading far quicker than efforts to contain the disease.

The WHO's chief, Margaret Chan, urged the presidents of Guinea, Liberia, Sierra Leone and Ivory Coast at a meeting in Guinea's capital Conakry to step up efforts to avoid "catastrophic consequences", after the global health organisation announced a $100m plan to combat an epidemic.

"This outbreak is moving faster than our efforts to control it. If the situation continues to deteriorate, the consequences can be catastrophic in terms of lost lives but also severe socioeconomic disruption and a high risk of spread to other countries."

"This meeting must mark a turning point in the outbreak response," Chan said.

The outbreak is by far the largest in the 40-year history of the disease, with 729 deaths so far, including more than 60 healthcare workers, and 1,323 cases overall, she said.

Experience showed that the outbreak could be stopped and the general public was not at high risk of infection, but it would be "extremely unwise" to let the virus circulate widely over a long period of time, she said.

Epidemiologists say the pathogen is relatively difficult to catch and is not airborne.

The virus, which requires contact with the bodily fluids of a victim, their blood, urine, faeces, vomit, saliva or sweat, to leap into a new host, is also treatable

Cultural setbacks

Chan said cultural practices such as traditional burials and deep-seated beliefs were a significant cause of the spread and a barrier to containment and needed to change.

People's assumption that isolation wards were "a sure death sentence" led them to care for loved ones at home or consult traditional healers, defeating attempts to contain the disease.

"Moreover, public attitudes can create a security threat to response teams when fear and misunderstanding turn to anger, hostility, or violence."

Despite alarm by some in the US, two American aid workers infected with the virus while in Liberia were set to arrive back home on Saturday.

Health officials said bringing the sickened aid workers into the country would not put the American public at risk.

On Thursday, Sierre Leone ordered a state of emergency over the Ebola epidemic and began setting up quarantines in parts of the country.

Sierra Leone's president, Ernest Bai Koroma said that the police and the military would restrict movement between affected areas, and would provide support to health officers and NGOs following a number of attacks on health workers by local communities.

Koroma also said that house-to-house searches would be implemented to trace Ebola victims and quarantine them.

He added that new protocols had been established for passengers arriving and departing Lungi airport outside Freetown.