At least $600m is needed to fight West Africa's Ebola outbreak, the World Health Organisation has announced, as the death toll shot up by about 400 in a week to more than 1,900 people.

There have now been about 3,500 cases in Liberia, Sierra Leone, Guinea, Nigeria and Senegal, WHO Director Margaret Chan confirmed on Wednesday.

The UN said that fresh fears emerged in Nigeria after the the government announced that a seventh person died of the virus.

"This is not an African disease. This is a virus that is a threat to all humanity,'' Gayle Smith, special assistant to US President Barack Obama and senior director at the National Security Council, told reporters.

Experts warn Ebola could spread beyond the five West African countries that are already hit.

The virus has directly impacted the economies of these nations with farmers and miners fearing that they will contract the infection if they step out to work in areas where cases were recorded.

The African Development Bank said that the outbreak could cost the affected countries up to four percent of their GDP.

Health workers infected

The top priority is providing protective gear to health workers in the affected areas and ensuring that they receive hazard pay, said David Nabarro, who is coordinating the UN response to the unprecedented outbreak.

A large number of health workers have been infected in this outbreak and many of them have expressed concern over the shortage of protective suits.

The key to solving the outbreak will be implementing measures used in all previous outbreaks: isolating and treating the sick, monitoring their contacts for signs of disease and safely burying the dead, said Tom Kenyon of the US Centers for Disease Control and Prevention.

"There is a window of opportunity but it's closing with each and every day that we delay in getting measures in place," he said.

Kenyon said experimental vaccines and treatments would not be available in time to make a difference.

On Tuesday, Doctors Without Borders (MSF) said that authorities were "losing the battle" against Ebola, and that the world had ignored the gravity of the epidemic.

The medical charity's president Joanne Liu called for a global biological disaster response, including funding for more field hospitals, trained civilian or military medical personnel and mobile laboratories in Guinea, Sierra Leone and Liberia.

Meanwhile, in the Democratic Republic of Congo, a separate strain of the virus killed at least 31 people, the government said. The WHO said that the Zaire strain is "distinct and independent" with no relationship to the outbreak in West Africa.