Image copyright EPA Image caption Health agencies have warned of an exponential surge in the number of Ebola cases in Liberia

More money has been announced to help the emergency response to the Ebola outbreak in West Africa.

The Gates Foundation is committing $50m to help step up efforts to tackle the deadly virus in the affected countries.

This comes on top of other funds announced by the UK and US governments, as well as the European Union.

But some aid charities say that the most urgent need in Africa is for expert teams in bio-hazard containment.

The Gates Foundation - set up by the Microsoft founder Bill Gates and his wife Melinda - says it will immediately release "flexible funds" to United Nations agencies and other organisations involved in the work against Ebola, so they can buy badly needed supplies.

And it says it will work with partners to speed up the development of drugs and vaccines against the virus, which has claimed almost 2,300 lives so far.

Nearly half of the deaths have been in Liberia. The country's defence minister has said it is facing a threat to its national existence.

And Sierra Leone's finance minister said the Ebola crisis had devastated the economy.

The CEO of the Gates Foundation, Sue Desmond-Hellmann, said: "We are working urgently with our partners to identify the most effective ways to help them save lives now.

"We also want to accelerate the development of treatments, vaccines and diagnostics that can help end this epidemic and prevent future outbreaks."

Wednesday's announcement is the latest financial commitment from international donors.

'Lethally inadequate'

Britain has already committed support worth $40m. Earlier this week, the UK's Department for International Development said it would set up a 62-bed medical treatment centre in Sierra Leone, to open within eight weeks.

Image copyright AFP Image caption A street artist paints a mural informing people of the symptoms of Ebola in the Liberian capital Monrovia

The European Union has announced funding worth $180m to help the governments in West Africa strengthen their health services - and to help local people by securing food and water supplies.

The US government has spent more than $100m in response to the outbreak. This includes funding for more than 100 extra African health workers to help run treatment units in Liberia, Guinea, Sierra Leone and Nigeria.

But the medical charity Medecins Sans Frontieres (MSF) has warned about a "lethally inadequate" international response, saying disaster response teams needed to be dispatched in collaboration with the affected African countries.

Its international president, Dr Joanne Liu, said last week: "While funding announcements, roadmaps, and finding vaccines and treatments are welcome, they will not stop the epidemic today.

"It is imperative that states immediately deploy civilian and military assets with expertise in biohazard containment."

According to the latest figures from the World Health Organization, there have been more than 4,000 cases in Guinea, Liberia and Sierra Leone.

In Nigeria, there have been 21 cases and 8 deaths. In Senegal, one case has been confirmed.

An official in Senegal said on Wednesday that the 21-year-old student who arrived from neighbouring Guinea last month had recovered.