High numbers of new cases of the Ebola virus are being reported in Sierra Leone and Liberia, with 19 deaths over three days this week, the UN's World Health Organization (WHO) says.

Such figures showed that it was a race against time to control the epidemic in Sierra Leone, medical charity MSF said.

In total there have been 539 deaths in West Africa since the outbreak began in neighbouring Guinea in February.

Regional leaders have now agreed to set up a fund to combat its spread.

At a summit of the regional body Ecowas in Ghana on Thursday, Nigeria's President Goodluck Jonathan agreed to contribute $3m (£1.8m) to the fund.

Ebola spreads through contact with an infected person's bodily fluids and there is no vaccine or cure.

It kills up to 90% of those infected but if patients receive early treatment, they have a better chance of survival.

'Precarious'

The WHO statement said that Sierra Leone had accounted for 32 new cases and 15 deaths, while Liberia reported 11 new cases and four deaths.

WHO: West Africa Ebola outbreak figures

• Guinea - 309 deaths, 409 cases
• Liberia - 88 deaths, 142 cases
• Sierra Leone - 142 deaths, 337 cases

There had been two deaths and one new infection recorded between 6-8 July in Guinea, where it said the community transmission rate was low.

"The epidemic trend in Liberia and Sierra Leone remains precarious with high numbers of new cases and deaths being reported," it said.

The cases in Sierra Leone are centred in Kailahun and Kenema districts, and in Liberia's Lofa and Montserrado counties, the WHO said.

The Ebola death rate in Guinea - where specialised health workers have been working since February - has slowed

Medecins Sans Frontieres said its teams in eastern Sierra Leone were "racing against time to stop the spread of the disease".

"We're under massive time pressure: the longer it takes to find and follow up with people who have come in contact with sick people, the more difficult it will be to control the outbreak," MSF emergency co-ordinator Anja Wolz said in a statement.

"We still have no idea how many villages are affected. I'm afraid we've only seen the tip of the iceberg."

The disease creates fear within communities and sick people are often stigmatised so experts believe the key to stopping the spread of the virus is to make sure affected communities understand it better.

"Families can be driven out of their villages, and sick people can be cast out to die on their own," said Ms Wolz.

The WHO gathers data on confirmed, probable and suspected cases and deaths in Guinea, Liberia and Sierra Leone.

So far in the West African outbreak there have been 888 cases.

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 the natural host of the virus