Two 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. His colleague Nancy Writebol arrived back in the city of Atlanta on Tuesday.
Since February, 887 people have died of Ebola in four West African countries.
US National Security Adviser Susan Rice said on Tuesday that the risk of Ebola transmission in the US is "very low".
"We have in this country the protocols to isolate and manage any patient who may present with those symptoms of the disease," she said.
Meanwhile, another man is being tested for Ebola at Mount Sinai Hospital in New York after travelling to West Africa.
The hospital said he was in isolation and "was stable overnight and in good spirits".
The World Bank is allocating $200m (£120m) in emergency assistance for countries battling to contain the Ebola outbreak.
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 eight people are currently in quarantine.
British Airways has temporarily suspended flights to and from Liberia and Sierra Leone until 31 August 2014 because of the health crisis, the airline said in a statement. It follows a similar suspension by two regional air carriers last week.
The virus spreads by contact with infected blood and bodily fluids. 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.
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.
According to a CNN report, quoting a doctor in Liberia, Dr Brantly's condition improved dramatically within an hour of receiving the drug.
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 in a special isolation ward at Atlanta's Emory University Hospital, where Dr Brantly is being treated by infectious disease specialists.
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.
In another development, the World Health Organization is to hold an emergency meeting in Geneva starting on Wednesday to discuss new measures to tackle the Ebola outbreak.
The meeting will decide whether to declare a global health emergency which could involve imposing travel restrictions on affected countries.
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Fatality rate can reach 90% - but the current outbreak is about 55%
- Incubation period is two to 21 days
- There is no vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats are considered to be virus' natural host