he Ebola outbreak in West Africa is spreading faster than efforts to control it, World Health Organization (WHO) head Margaret Chan has said.
She told a summit of regional leaders that failure to contain Ebola could be "catastrophic" in terms of lives lost.
But she said the virus, which has claimed 728 lives in Guinea, Liberia and Sierra Leone since February, could be stopped if well managed.
Ebola kills up to 90% of those infected.
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Ebola explained in 60 seconds
It spreads by contact with infected blood, bodily fluids, organs - or contaminated environments. Patients have a better chance of survival if they receive early treatment.
Initial flu-like symptoms can lead to external haemorrhaging from the eyes and gums, and internal bleeding that can lead to organ failure.
A US relief agency is repatriating two of its American staff who have contracted the virus in Liberia.
A flight carrying the first of the patients - Dr Kent Brantly - landed at and US Air Force base in Georgia at about 16:00 GMT.
Hundreds of US Peace Corps volunteers have already been evacuated from the West African countries.
Separately, US President Barack Obama announced that delegates from affected countries attending a US-Africa conference in Washington next week would be screened.
"Folks who 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," he said.
An employee of the Monrovia City Corporation sprays disinfectant inside a government building in a bid to prevent the spread of the deadly Ebola virus (1 August 2014)
Offices are being sprayed with disinfectant in the Liberian capital Monrovia to prevent the spread of the Ebola virus
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Analysis: David Shukman, BBC science editor
Friday's summit should provide the kind of international co-operation needed to fight Ebola but the battle against the virus will be won or lost at the local level. An over-attentive family member, a careless moment while burying a victim, a slip-up by medical staff coping with stress and heat - a single small mistake in basic hygiene can allow the virus to slip from one human host to another.
The basic techniques for stopping Ebola are well known. The problem is applying them. Since the virus was first identified in 1976, there have been dozens of outbreaks and all of them have been contained. Experts point to these successes as evidence that this latest threat can be overcome too.
But working against them are suspicions among local people and the unavoidable fact that this is an extremely poor part of the world, much of it still reeling from conflict. Deploying the right equipment in properly trained hands is always going to be a struggle, one that is now extremely urgent.
