LONDON — 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.