Early denial about the dangers of the contagious disease among Liberians has now "turned into fear and panic," she told CNN in an interview over Skype.
"There are dead bodies all over the place and they now know that it's real. They know that it's deadly and they are now beginning to respond," Ms Sirleaf said.
"This is very, very serious, it's very nearing a catastrophe."
The biggest challenge was trying to contain people in order to try to stop the spread of the epidemic.
After closing most of its land borders, Liberia announced Wednesday that it was shuttering all its schools and placing non-essential government staff on leave.
The impoverished country, along with neighboring Guinea and Sierra Leone, is struggling to contain an epidemic that has infected 1,200 people and left 672 dead across the region since the start of the year.
"This is not a Liberia problem or Sierra Leone problem or Guinea problem. It is an international problem," Ms Sirleaf insisted.
Monrovia was "grateful" for the people already on the ground, but needed more help to contain the spread and to train its doctors and nurses.
"They are stretched thin. They need to have additional support. Logistics, the supplies, preventive material... the things like chlorine and those things that will keep people safe," Sirleaf said.
Ms Sirleaf has canceled plans to visit Washington next week for a summit of African leaders hosted by US President Barack Obama and said she believed a US travel warning was an "acceptable preventive measure."