LONDON — Facing the worst known outbreak of the Ebola virus, with almost 1,000 fatalities in West Africa, the World Health Organization declared an international public health emergency on Friday, demanding an extraordinary response — only the third such declaration of its kind since regulations permitting such alarms were adopted in 2007.

The organization stopped short of saying there should be general international travel or trade bans, but acknowledged that the outbreak, already in its sixth month, was far from being contained.

One major international medical organization, Doctors Without Borders, responded to the statement with a renewed call for a “massive deployment” of health specialists to the stricken countries. “Lives are being lost because the response is too slow,” it said.

Dr. Margaret F. C. Chan, the director general of the World Health Organization, a United Nations agency, told a news conference at its Geneva headquarters, “This is the largest, most severe, most complex outbreak in the nearly four-decade history of the disease.”