Liberian president Ellen Johnson Sirleaf has begged the US for help in tackling Ebola, as American scientists warned that official forecasts on the development of the outbreak might be far too conservative.

The disease might not be contained for 18 months and could infect hundreds of thousands of people, epidemiologists mapping its spread for the US told the New York Times.

The World Health Organisation has forecast that Ebola could be contained in six to nine months after infecting around 20,000 people. But the American scientists warned that at the rate the disease is currently spreading, that many people could potentially be infected in a single month.

Dr Jeffrey L Shaman, associate professor of environmental health sciences at Columbia University Mailman School of Public Heath, was part of the project that forecast the outbreak would be contained in 12 to 18 months.

Ebola has infected more than 4,000 people in West Africa. Monrovia, Liberia's crowded capital, has been badly hit. A slum was quarantined last month, leading to food shortages and riots, and a teenager was killed when police opened fire on the crowds, but the blockade seemed to have little impact on the spread of Ebola.

Liberia is still recovering fromthe impact of a civil war that lasted over a decade and only ended in 2003. Without help in tackling the spread of Ebola, the country could slip back into chaos, Sirleaf warned in a letter to Barack Obama.

"I am being honest with you when I say that at this rate, we will never break the transmission chain and the virus will overwhelm us," the New York Times quoted her saying.