Ebola outbreak WHO

World Health Organization Assistant Director General Bruce Aylward called the Ebola crisis "unparalleled in modern times." (Denis Balibouse/Reuters)

The unprecedented Ebola outbreak in West Africa requires a $1 billion US response to keep its spread within the "tens of thousands" of cases, United Nations officials said on Tuesday.

The virus has killed 2,461 people, half of the 4,985 infected by the virus, and the toll has doubled in the last month, World Health Organization Assistant Director General Bruce Aylward said.

"Quite frankly, ladies and gentlemen, this health crisis we're facing is unparalleled in modern times," Aylward told a news conference in Geneva. "We don't know where the numbers are going on this."

He said the WHO's previous forecast that the number of cases could reach 20,000 no longer seemed a lot, but the number could be kept within the tens of thousands with "a much faster response".

U.N. Secretary-General Ban Ki-moon will launch a "global response coalition" in New York on Thursday, said Dr. David Nabarro, senior UN coordinator for Ebola.

"The amount for which we requested was about $100 million a month ago and now it is $1 billion, so our ask has gone up 10 times in a month," Nabarro told reporters.

"Because of the way the outbreak is advancing, the level of surge we need to do is unprecedented, it is massive," he said.

The United States announced on Tuesday that it would send 3,000 troops to help tackle the Ebola outbreak as part of a ramped-up response including a major deployment in Liberia, the country where the epidemic is spiralling fastest out of control.