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 today.

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,” said Dr. Bruce Aylward, WHO’s assistant director-general for emergency operations.

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