The World Health Organisation has declared the Ebola outbreak in West Africa to be an international public health emergency that requires an extraordinary response to stop its spread.

It is the largest and longest outbreak ever recorded of Ebola, which has a death rate of about 50 per cent and has so far killed at least 932 people. WHO declared similar emergencies for the swine flu pandemic in 2009 and for polio in May.

WHO chief Dr Margaret Chan said the announcement is "a clear call for international solidarity". Speaking at a news conference in Geneva she said: "Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own, I urge the international community to provide this support on the most urgent basis possible." The agency had convened an expert committee this week to assess the severity of the continuing epidemic.

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The current outbreak of Ebola began in Guinea in March and has since spread to Sierra Leone and Liberia, with a suspected cluster in Nigeria. There is no licensed treatment or vaccine for Ebola. WHO said countries without Ebola should heighten their surveillance and treat any suspected cases as a health emergency.

But the impact of the WHO declaration is unclear. "Statements won't save lives," said Dr Bart Janssens, director of operations for Doctors Without Borders. "For weeks, [we] have been repeating that a massive medical, epidemiological and public health response is desperately needed. Lives are being lost because the response is too slow." In the United States, the Centres for Disease Control and Prevention have elevated their Ebola response to the highest level and have recommended against travelling to West Africa.

American health officials have also eased safety restrictions on an experimental drug, a move that could clear the way for its use in patients. Two Americans infected with the virus recently received a drug never before tested in people and seem to be improving slightly, according to the charity they work for.