The top US public health official said that the epidemic would need an “unprecedented” response to bring it under control.
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.
Health ministers from across West Africa are due to meet in Ghana on Thursday to discuss the growing crisis.
The meeting comes after the African Development Bank warned that the outbreak is causing enormous economic damage to West Africa as foreign businesses quit the region.
Medical charity, Medecins Sans Frontieres (MSF), has, however, branded the international response “entirely inadequate”.
MSF operations director, Brice de la Vigne, said of the efforts to bring the outbreak under control; “It is simply unacceptable that serious discussions are only starting now about international leadership and coordination.
“Self-protection is occupying the entire focus of states that have the expertise and resources to make a dramatic difference.”
The World Health Organization says the outbreak, which has killed 1,427 people, 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.
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.