The World Health Organization (WHO) has declared the Ebola outbreak in West Africa to be an international public health emergency that requires an extraordinary response to stop its spread.
WHO declared similar emergencies for the swine flu pandemic in 2009 and for polio in May.
The WHO chief, Dr Margaret Chan, said the announcement on Friday is "a clear call for international solidarity" but acknowledged that many countries would probably not have any Ebola cases.
"Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own,'' Chan said at a news conference in Geneva.
"I urge the international community to provide this support on the most urgent basis possible," she added.
The agency had convened an expert committee this week to assess the severity of the ongoing epidemic - the largest and longest in history.
The current outbreak of Ebola began in Guinea in March and has since spread to Sierra Leone and Liberia.
The WHO said the situation is likely to get worse in the coming months.
There is no licensed treatment or vaccine for Ebola and the death rate has been about 50 percent.
The impact of the WHO declaration is unclear; the declaration about polio does not yet seem to have slowed the spread of virus.
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During a WHO meeting last week to reconsider the status of polio, experts noted countries had not yet fully applied the recommendations made in May, that there have been more instances of international spread and that outbreaks have worsened in Pakistan and Cameroon.
Meanwhile, Nigerian President Goodluck Jonathan declared a national state of emergency on Friday over the Ebola outbreak in Africa's most populous country.
He approved 1.9bn naira ($11.6m) of emergency funds to contain Ebola in the country, where seven cases have been confirmed since a man fell sick on arrival from Liberia, two of whom have died.
In the United States, the Centers for Disease Control and Prevention have already elevated their Ebola response to the highest level and has recommended against travelling to West Africa.
On Thursday, CDC director Dr Tom Frieden told a Congressional hearing that the current outbreak is set to sicken more people than all previous outbreaks of the disease combined.
"I don't know what the advantage is of declaring an international emergency," said Dr David Heymann, who directed WHO's response to the SARS outbreak and is now a professor at the London School of Hygiene and Tropical Medicine.
"This could bring in more foreign aid but we don't know that yet," he said.