Another US group, the Christian charity Samaritan's Purse, is also temporarily withdrawing non-essential staff from Liberia, citing regional instability and security issues.
A US State Department official said two volunteers were in isolation after having contact with a person who later died of the virus. The two were not showing symptoms of the disease and were under observation.
Liberia's President ordered the nation's schools to close and most civil servants to stay at home.
Ellen Johnson Sirleaf also called for the closure of markets in an area near the borders with infected countries Guinea and Sierra Leone. "My fellow Liberians, Ebola is real, Ebola is contagious and Ebola kills. Denying that the disease exists is not doing your part, so keep yourselves and your loved ones safe."
Ebola can kill victims in days, causing severe fever and muscle pain, vomiting, diarrhoea and organ failure and unstoppable bleeding. Since March, there have been 1201 cases of Ebola and 672 deaths in Guinea, Liberia and Sierra Leone, according to the World Health Organisation.
Sirleaf said security forces would enforce the new precautions taken a week after an American man of Liberian descent, Patrick Sawyer, boarded a plane in Monrovia and flew to Nigeria, where authorities said he died of Ebola. The fact that he was able to board a plane and travelled through a major airport transit hub in Togo has heightened fears about Ebola's possible spread in the region.
The airline involved, ASKY, has suspended its flights to both the capitals of Liberia and Sierra Leone, and said passengers leaving Guinea would be screened.
Experts say the risk of travellers contracting Ebola is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva. The virus cannot be spread like flu through casual contact or breathing the same air. Patients are contagious only once they they show symptoms.
"While the possibility of Ebola coming to Australia is very low, we are closely monitoring the overseas outbreak and Australia's domestic response," Australia's Chief Medical Officer, Professor Chris Baggoley, said. Border Protection agencies are on alert to look out for the possibility of Ebola when identifying a person landing in Australia who is unwell.
In Britain, Foreign Secretary Philip Hammond chaired the Government's Cobra crisis management committee to assess the situation. "The Prime Minister does regard it as a very serious threat," Hammond said. "We are very much focused on it as a new and emerging threat which we need to deal with."
The emergency meeting had decided the best approach was to provide "additional resources to deal with the disease at source" in west Africa.
The European Union was equipped and ready to treat victims should the deadly virus be found in its 28 member states, an EU source said in Brussels. "We cannot rule out the possibility that an infected person arrives in Europe but the EU has the means to track and contain any outbreak rapidly." The isolation and negative testing of a suspected case in Valencia in Spain showed that the system worked, the source said.
In Hong Kong, health officials said they would quarantine any visitors from Guinea, Sierra Leone and Liberia who showed fever symptoms. One woman arriving from Africa who showed symptoms including fever and vomiting tested negative for Ebola.
Medecins Sans Frontieres director of operation Bart Janssens warned that governments and global bodies had no "overarching view" on how to tackle the outbreak. "This epidemic is unprecedented, absolutely out of control and the situation can only get worse, because it is still spreading."
The scientist who helped discover the virus says the outbreak is unlikely to trigger a major epidemic outside the region. But Professor Peter Piot said that a "really bad" sense of panic and lack of trust in the authorities in west Africa had contributed to the outbreak. The Belgian scientist urged officials to test experimental vaccines on people with the virus so that when it inevitably returns, the world is prepared.
- AFP, AP, AAP
