WASHINGTON, Sept. 23 -- The number of Ebola cases in West Africa could reach as many as 550,000 to 1.4 million by Jan. 20 in the worst-case scenario, according to a report released Tuesday by the U.S. Centers for Disease Control and Prevention ( CDC).

But the CDC report was also quick to note that the higher projections are "very unlikely" since measures to contain the outbreak are "rapidly implemented and sustained."

The U.S. agency developed a new modeling tool called EbolaResponse to estimate the potential number of future cases. The report was based on data from August but did not account for the ongoing international Ebola relief efforts.

In the worst-case scenario, the CDC researchers assumed that the Ebola cases are significantly underreported by a factor of 2.5 in Sierra Leone and Liberia, two of the three hardest-hit countries in West Africa.

Using this correction factor, the report estimated that approximately 21,000 total cases will have occurred in Liberia and Sierra Leone by Sept. 30.

Reported cases in Liberia are doubling every 15 to 20 days, and those in Sierra Leone are doubling every 30 to 40 days, it said.

If trends continued without additional interventions or changes in community behavior, such as notable reductions in unsafe burial practices, Liberia and Sierra Leone will have a range of between 550,000 and 1.4 million Ebola cases by Jan. 20, it projected.

In the best-case model, however, the epidemic "would almost be ended" by Jan. 20 if approximately 70 percent of persons with Ebola are treated in medical facilities and the dead are safely buried, the report said.

"The model shows that there are severe costs of delay, and the need for increased resources and immediate and ongoing action by the international community," CDC Director Tom Frieden said.

"It is still possible to reverse the epidemic, and we believe this can be done if a sufficient number of all patients are effectively isolated, either in Ebola Treatment Units or in other settings, such as community-based or home care."

He said once a sufficient number of Ebola patients are isolated, cases will "decline very rapidly -- almost as rapidly as they rose. "

The World Health Organization published its own revised estimates of the outbreak Tuesday, predicting more than 20,000 cases by Nov. 2 unless control measures are enhanced quickly.

As of Sept. 19, the total number of cases attributed to Ebola virus disease in Guinea, Liberia, and Sierra Leone has reached 5, 843, including 2,803 deaths.