GENEVA — The Ebola outbreak in West Africa eventually could exceed 20,000 cases, more than six times as many as are now known, the World Health Organization said Thursday.

A new plan by the U.N. health agency to stop Ebola also assumes that the actual number of cases in many hard-hit areas may be two to four times higher than currently reported. If that’s accurate, it suggests there could be up to 12,000 cases already.

“This far outstrips any historic Ebola outbreak in numbers. The largest outbreak in the past was about 400 cases,” Dr. Bruce Aylward, WHO’s assistant director-general for emergency operations, told reporters.

“What we are seeing today, in contrast to previous Ebola outbreaks: multiple hotspots within these countries — not a single, remote forested area, the kind of environments that have been tackled in the past. And then not multiple hotspots within one country, but international disease.”

Another new dimension, he said, is the difficulty of dealing with Ebola in large cities and broad areas.

The agency published new figures saying that 1,552 people have died from the killer virus from among the 3,069 cases reported so far in Liberia, Sierra Leone, Guinea and Nigeria. At least 40 percent of the cases have been identified in the last three weeks, the U.N. health agency said, adding that “the outbreak continues to accelerate.”

In Geneva, the agency released a new plan for handling that aims to stop Ebola transmission in affected countries within six to nine months and prevent it from spreading internationally.

The plan calls for $489 million to be spent over the next nine months and requires 750 international workers and 12,000 national workers.

The 20,000 cases figure, said Aylward, “is a scale that I think has not ever been anticipated in terms of an Ebola outbreak.”

“That’s not saying we expect 20,000,” he added. “But we have got to have a system in place that we can deal with robust numbers.”

Aylward said the agency is urging airlines to lift most of their restrictions about flying to Ebola-hit nations because a predictable air link is needed to help deal with the crisis. Air France on Wednesday canceled its flights to Sierra Leone.

Nigerian authorities, meanwhile, said a man who contracted Ebola after coming into contact with a traveler from Liberia had evaded surveillance and infected a doctor in southern Nigeria who later died.