The World Health Organization on Friday 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 percent 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.

The WHO chief, Dr. Margaret Chan, said the announcement is "a clear call for international solidarity" although she 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."

The agency had convened an expert committee this week to assess the severity of the continuing epidemic.

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.

The impact of the WHO declaration is unclear; the declaration about polio doesn't yet seem to have slowed the spread of virus.

"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 Centers for Disease Control and Prevention have already elevated their Ebola response to the highest level and have recommended against traveling 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.

Other experts hoped the declaration would send more health workers to West Africa.

"The situation is very critical and different from what we've seen before," said Dr. Heinz Feldmann, chief of virology at the U.S. National Institute of Allergy and Infectious Disease. "There are so many locations with transmission popping up and we just need more people on the ground."

WHO did not recommend any travel or trade bans but said people who had close contact with Ebola patients should not travel internationally. For countries with Ebola, WHO issued various recommendations, including exit screening at international airports and border crossings to spot potential cases. It also discouraged mass gatherings.

WHO said countries without Ebola should heighten their surveillance and treat any suspected cases as a health emergency.

This week, two of the worst-hit Ebola countries -- Liberia and Sierra Leone -- brought in troops to enforce quarantines and stop people infected with the disease from traveling. Liberian President Ellen Johnson Sirleaf said no one with a fever would be allowed in or out of the country and warned some civil liberties could be suspended if needed to bring the virus under control.

Chan said while extraordinary measures might be necessary to contain the outbreak, it is important to recognize civil rights.

"We need to respect the dignity of people and inform them why these measures are being taken," she said.