The worst-ever outbreak of Ebola fever has now killed more than 2,400 people and infected twice that number, according to a new toll released on Friday by the World Health Organization.

"As of 12 September, we are at 4,784 cases and more than 2,400 deaths," the head of the UN health agency, Margaret Chan, told a news conference on the spiralling west African health crisis.

"In the three hardest hit countries, the number is moving faster than the capacity to manage them," she warned, calling for a stronger, faster response from the international community.

In Liberia, Chan said there is not a single bed left to treat Ebola patients.

Chan did not specify if the latest toll included Nigeria, in addition to Liberia, Guinea and Sierra Leone, the three nations at the epicentre of the epidemic.

The WHO's previous toll on Tuesday reported 2,300 death across the four countries since the start of the year.

Cuba's health minister announced at a news conference with Chan earlier Friday that it would be sending send 165 doctors and nurses to Sierra Leone for six months to help fight Ebola.

Ebola, transmitted through bodily fluids, leads to haemorrhagic fever and -- in over half of cases -- death. There is no specific treatment regime and no licensed vaccine.