LONDON — New estimates by the World Health Organization and the U.S. health agency are warning that the number of Ebola cases could soar dramatically unless efforts to curb the outbreak are significantly ramped up.

The United States says there could be up to 1.4 million by mid-January in two nations alone.

Since the first cases were reported six months ago, the tally of cases in West Africa has reached an estimated 5,800 illnesses and over 2,800 deaths. But the U.N. health agency has warned that tallies of recorded cases and deaths are likely to be gross underestimates of the toll that the killer virus is wreaking on West Africa.

The U.N. health agency said today that the true death toll for Liberia, the hardest-hit nation in the outbreak, may never be known, since many bodies of Ebola victims in a crowded slum in the capital, Monrovia, have simply been thrown into nearby rivers.

In its new analysis, WHO said Ebola cases are rising exponentially and warned the disease could sicken people for years to come without better control measures. The WHO’s calculations are based on reported cases only.

However, the U.S. Centers for Disease Control and Prevention released its own predictions today for the epidemic’s toll, based partly on the assumption that Ebola cases are being underreported. The report says there could be up to 21,000 reported and unreported cases in Liberia and Sierra Leone alone by the end of this month and that cases could balloon to as many as 1.4 million by mid-January.

Experts caution those predictions don’t take into account response efforts.