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 729 lives in four West African countries 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 areas like eyes and gums, and internal bleeding which can lead to organ failure.
Dr Chan was meeting the leaders of the worst-affected countries - Guinea, Liberia and Sierra Leone - to launch a new $100m (£59m; 75m euro) Ebola response plan.
The plan includes funding the deployment of hundreds more health care workers to affected countries.
Friday's summit should provide the kind of international cooperation 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.
"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, explaining that the outbreak was the world's deadliest and largest in terms of geographical areas.
"It is taking place in areas with fluid population movements over porous borders, and it has demonstrated its ability to spread via air travel, contrary to what has been seen in past outbreaks," she said.
In her comments - also published on the WHO website - Dr Chan said the virus was affecting a large number of doctors, nurses and other health care workers who have an essential role in curtailing the outbreak.
"To date, more than 60 health care workers have lost their lives in helping others. Some international staff are infected. These tragic infections and deaths significantly erode response capacity," she said.
Dr Chan said that while the situation in West Africa "must receive urgent priority for decisive action at national and international levels, experiences in Africa over nearly four decades tell us clearly that, when well managed, an Ebola outbreak can be stopped".
She pointed out that medics are not fighting an airborne virus - transmission requires close contact with the bodily fluids of an infected person.
"Apart from this specific situation, the general public is not at high risk of infection by the Ebola virus," she said.
"At the same time, it would be extremely unwise for national authorities and the international community to allow an Ebola virus to circulate widely and over a long period of time in human populations."
She also said that:
* Constant mutation and adaptation are the survival mechanisms of viruses and it was important not to allow opportunities for them to deliver more surprises
* Medics are not just up against a public health problem but also a social problem made worse by "deep-seated beliefs and cultural practices"
* Chains of transmission have moved underground, are invisible and are not being reported
* Because of the high fatality rate, many people prefer to care for loved ones in their homes which hampers rapid containment of the virus
* Public attitudes can create a security threat to response teams when fear and misunderstanding turn to anger, hostility or violence
Separately, the Liberian government declared Friday a non-working holiday, during which public and private places of work "will be shut down" to allow a huge sanitization and chlorination exercise in government ministries and places of public gathering.
Information Minister Lewis Brown told the BBC that "everywhere is shut down".
He said earlier on state radio that "the intent is to let us come to the realisation that something is wrong and what is wrong is serious".
* 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