A week after the World Health of Organization announced that fighting the Ebola virus outbreak in West Africa would cost $600 million, the United Nations General Secretary Ban Ki-Moon announced it was in need of $1 billion to do so.

"This is a disease outbreak that is advancing in an exponential fashion," David Nabarro, special envoy to West Africa, told Bloomberg. "I estimate that to get ahead of outbreak, the level of response needs to be about 20 times greater than it is at the moment."

The outbreak has currently killed more than 2,600 individuals and more than 5,000 cases have been reported since it was first reported earlier this year.

On Sept. 12, the U.N. announced a WHO plan to stop the transmission of the virus, and estimated that of the $600 million needed, at least $490 million was needed specifically to fund the plan. Recently, they received a pledge for as much as $50 million from the Gates Foundation -- the largest donation yet by the foundation.

Of the total $988 million sought, only 30 percent has been received by the U.N.

The virus had spread to three countries -- Liberia, Guinea and Sierra Leone -- but Nigeria and most recently Senegal are also being widely affected, Bloomberg reported.

The almost $1 billion sought would be spent over the next six months, with a strategy aiming to begin the mission by the end of the month.

The U.S. has already pledged, on Sept. 16, to send 3,000 troops to the region and help build 20 100-bed treatment centers, and train about 500 health care providers, Bloomberg reported.

The U.N. has received criticism for its apparently slow response to the outbreak, with the first major address of the situation coming in August -- months after the outbreak was reported in the Spring.

No cure for the disease exists but experimental drugs have been relatively successful, and have been green-lighted -- along with experimental vaccines -- to move to clinical trials and be produced as part of an emergency response.