The Ebola outbreak that has already claimed more than 1000 lives in west Africa is moving faster than aid organisations can handle, the medical charity MSF says.
The warning came just a day after the World Health Organisation said the scale of the epidemic had been vastly underestimated and that "extraordinary measures" were needed to contain the killer disease.
The UN health agency said the death toll from the worst outbreak of the disease in four decades had now climbed to 1069 in four afflicted countries.
"It is deteriorating faster, and moving faster, than we can respond to," the head of MSF Joanne Liu told reporters in Geneva.
"It is like wartime," she said.
WHO had said in a statement late on Thursday it was co-ordinating "a massive scaling up of the international response", in a bid to tackle the epidemic that has claimed lives in Guinea, Liberia, Nigeria and Sierra Leone.
"Staff at the outbreak sites see evidence that the numbers of reported cases and deaths vastly underestimate the magnitude of the outbreak," it said.
"The outbreak is expected to continue for some time. WHO's operational response plan extends over the next several months."
As countries around the world stepped up measures to contain the disease, the International Olympics Committee said athletes from Ebola-hit countries have been barred from competing in pool events and combat sports at the Youth Olympics opening in China on Saturday.
The decision, which affects three unidentified athletes, was made "with regard to ensuring the safety of all those participating" in the Games in the city of Nanjing, the IOC and Chinese organisers said in a joint statement.
No cure or vaccine is currently available for Ebola, which the WHO has declared a global public health emergency and subsequently authorised the use of largely untested treatments in efforts to combat the disease.
Hard-hit countries are awaiting consignments of up to 1000 doses of the barely tested drug ZMapp from the United States, which has raised hopes of saving hundreds infected with the disease.
Canada says between 800 and 1000 doses of a vaccine called VSV-EBOV, which has shown promise in animal research but never been tested on humans, would also be distributed through the WHO.
The last days of an Ebola victim can be grim, with the most severe cases succumbing to agonising muscular pain, vomiting, diarrhoea and catastrophic haemorrhaging described as "bleeding out" as their organs break down.
The cost of tackling the virus is also threatening to exact a severe economic toll on the already impoverished west African countries at the epicentre of the epidemic.
"The outbreak risks having a direct financial effect on government budgets via increased health expenditures that could be significant," Moody's rating agency said.
A serious outbreak in Lagos, where Ebola has claimed four lives, could severely disrupt the oil and gas industry in Nigeria if international companies are forced to evacuate staff and local operations are shut down, Moody's warned.
Across the region, increasingly draconian restrictions have been put in place.
Guinea, where at least 377 people have died, declared a health emergency on Wednesday and ordered strict controls at border points and a ban on moving bodies from one town to another.
A number of airlines have cancelled flights in and out of west Africa, while Gambia became the latest country to suspend flights from Guinea, Liberia and Sierra Leone
Although the WHO confirmed that other African countries, including Kenya, were labelled "high risk" due to their popular transport hubs, it also emphasised that air travel, even from Ebola-affected countries, is low risk because the virus is not airborne.
