The 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.
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.
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.
Ebola since 1976
Dr Chan met the leaders of Guinea, Liberia and Sierra Leone to launch a new $100m (£59m) Ebola response plan.
The plan includes funding the deployment of hundreds more health care workers to affected countries.
"This meeting must mark a turning point in the outbreak response," Dr Chan said at the summit in Guinea's capital, Conakry.
"Cases are occurring in rural areas which are difficult to access, but also in densely populated capital cities."
She said the outbreak was the deadliest and most widely spread, and had also demonstrated an ability to spread through air travel, unlike past outbreaks.
Separately, the Liberian government declared Friday a holiday to allow a huge sanitisation and chlorination exercise in government ministries and places of public gathering.
Information Minister Lewis Brown said "the intent is to let us come to the realisation that something is wrong and what is wrong is serious".
Up to 30 Commonwealth Games athletes from Sierra Leone, meanwhile, are considering extending their stay in Glasgow amid fears over the Ebola virus.
Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Fatality rate can reach 90%
- Incubation period is two to 21 days
- There is no vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats are considered to be virus' natural host