An Ebola outbreak that has killed more than 700 people in West Africa is moving faster than efforts to control the disease, the head of the World Health Organization warned as presidents from the affected countries met Friday in Guinea’s capital.

Dr. Margaret Chan, the WHO’s director-general, said the meeting in Conakry “must be a turning point” in the battle against Ebola, which is now sickening people in three African capitals for the first time in history.

At least 729 people have died since cases first emerged in March: 339 in Guinea, 233 in Sierra Leone, 156 in Liberia and one in Nigeria.

Two American health workers in Liberia have been infected, and an American man of Liberian descent died in Nigeria from the disease, health authorities there say.

Plans were underway to bring the two American aid workers ― Nancy Writebol and Dr. Kent Brantly ― back to the U.S. A small private jet based in Atlanta has been dispatched to Liberia. Officials said the jet was outfitted with a special, portable tent designed for transporting patients with highly infectious diseases.

While health officials say the virus is transmitted only through direct contact with bodily fluids, many sick patients have refused to go to isolation centers and have infected family members and other caregivers.