More than 120 health workers have died of Ebola across west Africa, the World Health Organisation has said, claiming that the epidemic had affected an "unprecedented number of medical staff".

In a statement on Monday, the WHO said more than 240 healthcare workers working in Guinea, Liberia, Nigeria, and Sierra Leone had developed the disease with "more than 120" succumbing to the epidemic.

The announcement came as Japan said it was ready to provide a Japanese-developed anti-influenza drug as a potential treatment to fight Ebola.

Yoshihide Suga, the chief cabinet secretary, said on Monday that Tokyo could offer the tablet favipiravir any time at the request of the WHO.

In case of an emergency, Japan could respond to individual requests before any further decision by the WHO, he said.

The WHO said earlier this month that it was ethical to use untested drugs on Ebola patients given the magnitude of the outbreak.

The drug was approved by the Japanese health ministry in March to treat cases of influenza.

The company behind the drug, Fujifilm, is talking to the US food and drug administration on clinical testing of the drug in treating Ebola.

The company has favipiravir stock for more than 20,000 patients, Aoki said.

A company spokesman said Ebola and influenza viruses were the same type and theoretically similar effects could be expected on Ebola.

Several drugs are being developed for Ebola, but they are still in early stages and there is no proven treatment or vaccine for the highly fatal disease.

Ebola has killed more than 1,400 people in West Africa in the latest outbreak.