US aid workers who contracted Ebola in Liberia appear to be improving after receiving an experimental drug, officials have said.
But it is not clear if the ZMapp drug, which has only been tested on monkeys, can be credited with their improvement.
Dr Kent Brantly was flown home for treatment on Saturday and his colleague Nancy Writebol left Liberia in the early hours of Tuesday.
Since February, 887 people have died of Ebola in four West African countries.
The World Bank is allocating $200m (£120m) in emergency assistance for countries battling to contain the Ebola outbreak.
Dr Anthony Fauci on hopes for an Ebola vaccine
It is the world’s deadliest outbreak to date and has centred on Guinea, Liberia and Sierra Leone. There have also been two cases in the Nigerian city of Lagos, where another eight people are currently in quarantine.
The virus spreads by contact with infected blood and bodily fluids and the current outbreak is killing between 50% and 60% of people infected.
There is no cure or vaccine for Ebola – but patients have a better chance of survival if they receive early treatment.
Nigeria health officials wait to screen passengers at the arrival hall of Murtala Muhammed International Airport in Lagos, Nigeria, Monday, 4 August 2014 Airports in Nigeria are now screening passengers for Ebola on arrival
A person in a protective suit works at an Ebola isolation ward at a mission hospital outside of Monrovia, Liberia (4 August 2014) Health workers have to wear protective suits to ensure they do not catch the virus
Dr Kent Brantly at the case management centre on the campus of ELWA Hospital in Monrovia Kent Brantly was able to walk from the ambulance when he arrived at the Atlanta hospital on Saturday
Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, says clinical trials are to start in September on an Ebola vaccine that has shown promising results during tests on animals.
“By the middle to end of 2015 we’ll be able to have some vaccine – at least to vaccinate health workers – who put themselves at considerable risk when they take care of these patients,” he told the BBC’s Newsday programme.
The US aid workers were treated with the ZMapp serum before their evacuation from Liberia.
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Service in Mission (SIM), the Christian aid group that employs Ms Writebol, says she has had two doses of the drug and did not respond as well as Dr Brantly but she is showing “improvement”.
“She is walking with assistance… strength is better… has an appetite,” SIM spokesman Palmer Holt told the Washington Post newspaper in an email on Monday.
Ms Writebol is on her way to a special isolation ward at Atlanta’s Emory University Hospital, where Dr Kent Brantly is being treated by infectious disease specialists.
Map showing Ebola outbreaks since 1976
Three companies, the US government and the Public Health Agency of Canada are behind the experimental drug.
“ZMapp was first identified as a drug candidate in January 2014 and has not yet been evaluated for safety in humans. As such, very little of the drug is currently available,” Mapp Biopharmaceutical said in a statement.
“Mapp and its partners are co-operating with appropriate government agencies to increase production as quickly as possible.”
Ebola has initial flu-like symptoms that can lead to external haemorrhaging from areas like eyes and gums, and internal bleeding which can lead to organ failure.
African leaders attending the first-ever US-Africa summit are expected to discuss the crisis in Washington.