Washington (AFP) - Two Americans infected with Ebola in West Africa will be evacuated back to the United States in the coming days to be cared for in strict isolation, officials said Friday.

Kent Brantly, a doctor who was treating Ebola patients in Liberia, and Christian missionary worker Nancy Writebol, are being flown home, but it was not immediately clear when they would arrive back in the US.

Both are in serious but stable condition and are headed for special care isolation units at a hospital in Atlanta, Georgia, after battling for their lives amid the worst outbreak of the virus in history.

Meanwhile US President Barack Obama said that delegates from Ebola-hit countries attending a US-Africa summit next week in Washington and who may have been exposed to the deadly virus would be screened before leaving their countries.

The hemorrhagic fever has killed 729 people of the more than 1,300 infected since March.

The World Health Organization said the fast-moving outbreak was causing "catastrophic" loss of life in the affected countries of Guinea, Liberia, Nigeria and Sierra Leone.

Obama said that Washington was confident in the measures it had put in place as he prepares to welcome about 50 heads of state and government to next week's summit.

"Folks that are coming from these countries that have even a marginal risk, or an infinitesimal risk of having been exposed in some fashion, we're making sure we're doing screening on that end as they leave the country," Obama told reporters.

He added there would be "additional screening" when they were in the United States.

"The safety and security of US citizens is our paramount concern," State Department deputy spokeswoman Marie Harf said, confirming her agency was facilitating the medical evacuation of the two infected Americans 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."

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Both patients will be sent to Emory University Hospital, though they may arrive at separate times, according to Bruce Ribner, an infectious disease specialist.

In contrast to where they are currently receiving treatment, "we can deliver a substantially higher level of care and a substantially higher level of support to optimize the likelihood that those patients will survive this episode," he told a news conference.

The hospital said it has a "specially built isolation unit" made for patients with certain serious infectious diseases and which is one of only four such facilities in the country.

Patients would be arriving in a non-military plane at Dobbins Air Reserve Base in Georgia, the Pentagon said.

"We are grateful that Kent Brantly and Nancy Writebol made it through the night," said Franklin Graham, president of Samaritan's Purse, the group for which Brantly worked.

"They remain in serious condition. The medical evacuations could be complete early in the week."

CDC chief Tom Frieden said 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."

"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," he told CNN.

The World Health Organization meanwhile 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 infected to more than 1,300 since the epidemic began earlier this year.