The head of the World Health Organization says the Ebola outbreak in West Africa is out of control.

WHO chief Margaret Chan warned today that the outbreak is moving faster than international efforts to control it.

Chan met today with the presidents of Guinea, Liberia, Sierra Leone and Ivory Coast to talk about the worst outbreak in the history of the disease. She told the African leaders the disease can be stopped, but only if the right steps are taken immediately.

Meantime, plans are underway to bring back the two American aid workers sick with Ebola from Africa.

A small private jet based in Atlanta has been dispatched to Liberia where the two Americans work for missionary groups. Officials say the jet is outfitted with a special, portable tent designed for transporting patients with highly infectious diseases.

The plane can only carry one patient at a time. Atlanta's Emory University Hospital says it expects one patient to be transferred there ``within the next several days.'' The hospital has a special isolation unit.

The two Americans are Dr. Kent Brantly and Nancy Writebol. Both worked at a hospital that treats Ebola patients.

The Ebola outbreak in West Africa has killed more than 700 people.