Two US citizens infected by the Ebola virus in West Africa will be evacuated back to the United States to be cared for in strict isolation in the coming days, the State Department said Friday.

"The safety and security of US citizens is our paramount concern," deputy spokeswoman Marie Harf said, confirming the State Department was facilitating the medical evacuation with the Centers for Disease Control and Prevention (CDC).

"Every precaution is being taken to move the patients safely and securely, to provide critical care en route on a non-commercial aircraft and to maintain strict isolation upon arrival in the United States."

They will be "taken to medical facilities with appropriate isolation and treatment capabilities," Harf added in her statement.

Samaritan's Purse, a US charity, has said that two of its staff members, doctor Kent Brantly and another American missionary worker, Nancy Writebol, were stricken with the virus in Liberia.

Both are in "serious" condition, aid groups said Friday. SIM USA, the Christian group for which Writebol worked, said she was in "serious but stable" condition.

"We are so heartened that Nancy is in stable condition and that plans are underway to bring her back to the US," said Bruce Johnson, president of SIM USA.

Emory University Hospital, in southern Georgia, has also said that it is preparing to receive "a patient with Ebola virus infection to its special facility containment unit within the next several days."

"CDC protocols and equipment are used for these kinds of medical evacuations so that they are carried out safely," Harf added.

This was done in an effort to protect "the patient and the American public, as has been done with similar medical evacuations in the past," she added.

Brantly, 33, became infected with Ebola while working with patients in the Liberian capital of Monrovia as he helped treat victims of the worst Ebola outbreak in history.

CDC chief Tom Frieden said that the decision to evacuate the two Americans was made by the aid groups for which they worked, not the CDC, though the agency "want(s) to support them in their decision."

"One thing we've traditionally said is that if someone has Ebola, the risk of medical evaluation may well outweigh the benefits of a higher level of medical care," Frieden told CNN.

"A flight over the ocean is arduous. Someone who has Ebola may have delicate arteries and veins. That may lead to bleeding complications. So travel itself may be harmful."

Samaritan's Purse said "the two Americans who contracted Ebola in Liberia remain in the country today but medical evacuation efforts are underway and should be completed by early next week."

A Pentagon spokesman said "the plane will be arriving at Dobbins Air Base in Georgia," but gave no details on the timing, and said the pair would not be aboard a military aircraft.

The World Health Organization warned West Africa's Ebola-hit nations that the epidemic was spiraling out of control and could spread to other countries.

The WHO raised the death toll by 57 to 729 on Thursday, announcing that 122 new cases had been detected between Thursday and Sunday last week, bringing the total to more than 1,300 since the epidemic began earlier this year.