The number of new Ebola cases in West Africa is growing faster than authorities can manage them, the World Health Organization (WHO) said on Friday, renewing a call for health workers from around the world to go to the region to help.

As the death toll rose to more than 2,400 people out of 4,784 cases, WHO director general Margaret Chan told a news conference in Geneva the vast nature of the outbreak -- particularly in the three hardest-hit countries of Guinea, Liberia and Sierra Leone -- required a massive emergency response.

Sarah Crowe, a spokeswoman for UNICEF, said the U.N. children's agency was using innovative ways to tackle the epidemic, including telling people to "use whatever means they have, such as plastic bags, to cover themselves if they have to deal with sick members of their family".

"The Ebola treatment centers are full, there are only three in the country. Families need help in finding new ways to deal with this and deal with their loved ones and give them care without exposing themselves to this infection," she said via phone from Monrovia.

"It is quite surreal and everywhere there is a sense of this virus taking over the whole country," Crowe said. "We do not have enough partners on the ground. Many Liberians say they feel abandoned."

Survivors of the disease, who are immune to reinfection, were being used to look after thousands of children of people with suspected Ebola. About 2,000 children have lost one or both parents in Liberia alone, she said.

The key to beating the disease, said the WHO's Chan, was people power. Pledges of equipment and money are coming in, but 500-600 foreign experts and at least 1,000 local health workers are needed on the ground.

"The number of new patients is moving far faster than the capacity to manage them. We need to surge at least three to four times to catch up with the outbreaks," Chan said.

CUBA HELPS

Cuban Health Minister Roberto Morales Ojeda, sitting alongside Chan, said his country would send 165 healthcare workers to help in the fight - the largest contingent of foreign doctors and nurses to be committed so far. However, they will arrive in October and will go to Sierra Leone, while thousands of new patients are expected in Liberia within weeks.

Chan said the real death toll is probably far higher than the latest number of 2,400.

"We are very cognizant of the fact that any number of cases and deaths that we are reporting is an underestimate." she said.

The Ebola infection rate and death toll have been particularly high among health workers, who are exposed to hundreds of highly infectious patients who can pass the virus on through body fluids such as blood and excrement.

Almost half of the 301 healthcare workers who have developed the disease have died.

Some foreign healthcare workers in West Africa, including several Americans and at least one Briton, have also been infected. Two Dutch doctors who may have been exposed to the disease in Sierra Leone are set to be evacuated.

Chan's call chimed with pleas from leading Ebola specialists, including Peter Piot, one of the scientists who first identified the Ebola virus in 1976.

Writing in the online scientific journal Eurosurveillance with his colleague Adam Kucharski, Piot, now director of the London School of Hygiene and Tropical Medicine, said it was hard to track an outbreak with exponential growth in case numbers.

"There are currently hundreds of new Ebola virus disease cases reported each week; with the number of infections increasing exponentially, it could soon be thousands," they said, adding that case numbers could double every fortnight.

"Fear and mistrust of health authorities has contributed to this problem, but increasingly it is also because isolation centers have reached capacity. As well as creating potential for further transmission, large numbers of untreated - and therefore unreported - cases make it difficult to measure the true spread of infection, and hence to plan and allocate resources."

The U.N. health agency has previously warned there could be as many as 20,000 cases in the region before the outbreak is brought under control.

In a glimmer of good news, the WHO said eight districts with previous Ebola cases - four in Guinea, three in Sierra Leone and one in Liberia - had reported no new cases for three weeks.

And 67 people who had contact with a person who had taken the disease to Senegal on Aug 20 had been traced, and none had so far tested positive for the disease.

The International Monetary Fund said on Thursday that economic growth in Liberia and Sierra Leone could decline by as much as 3.5 percentage points due to the outbreak, which it said has crippled their mining, agriculture and services sectors.

(Writing by Tom Miles and Kate Kelland; Editing by Sonya Hepinstall)