"Lofa county in the north has been quarantined by the army," Sirleaf said after similar measures were taken in the provinces of Boma and Grand Cape Mount.
"So from now on, no one will be allowed to go to Lofa, no one will come out of there," Sirleaf said in a message to the nation broadcast over the radio.
"We want to protect areas that have not been yet affected," according to the president.
"We have given instructions to the army and we will try our best so that food will be provided to the county constantly, especially when we know that Lofa is one of (the) biggest counties."
In an effort to contain an epidemic that has cost nearly 1,000 lives throughout west Africa, the Liberian government earlier quarantined Boma and Grand Cape Mount, the most affected northern provinces.
It cut off access with military roadblocks and restrictions on travel.
The quarantine has meant that traders have been unable to travel to buy food and farmers cannot harvest their crop, which has in turn caused shortages and sent prices soaring, raising fears people could go hungry.