Tom Frieden, the director of the Centers for Disease Control, said the epidemic would need an "unprecedented" response to bring it under control.
Health ministers from across West Africa are due to meet in Ghana to discuss the growing crisis.
The World Health Organization says the outbreak has killed 1,427 people.
The health body says it is the largest ever Ebola epidemic and has infected an estimated 2,615 people.
Liberia has been hardest-hit of the affected countries, with 624 deaths and 1,082 cases since the start of the year.
Mr Frieden met Liberian President Ellen Johnson Sirleaf to discuss ways to fight the disease.
"The cases are increasing. I wish I did not have to say this, but it is going to get worse before it gets better," he admitted.
"The world has never seen an outbreak of Ebola like this. Consequently, not only are the numbers large, but we know there are many more cases than has been diagnosed and reported," he added.
He said there was a need for "urgent action" and called on Liberians "to come together" to stop misconceptions that have helped the outbreak spread.
Despite rumours to the contrary, the virus is not airborne and is spread by humans coming into contact with bodily fluids, such as sweat and blood, from those infected with virus.
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