The country that was hit hardest at the peak of the deadly epidemic has seen more than 4,000 deaths, but was at an advanced stage in its recovery and was expecting to be declared Ebola-free by mid-April.
"A woman has been confirmed as an Ebola patient... This is a new case after we have gone more than 27 days without a single case. It is a setback," government spokesman Lewis Brown told AFP yesterday.
It was not immediately clear where the new patient became infected, as all contacts associated with the last known chain of transmission have completed the 21-day observation period, according to the WHO.
Surveillance and early warning systems detected 125 suspected cases in the week to March 15 but none tested positive for Ebola.
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Since the outbreak began in December 2013, 24,753 people in nine countries have been infected with the virus, and 10,236 of them have died, according to the latest figures.
But the tide seemed to have turned in Liberia, which six months ago was reporting more than 300 new cases a week and which still counts the most deaths in the outbreak, at 4,283.
The country discharged its last confirmed Ebola patient, Beatrice Yordoldo, on March 5, two days after she tested negative for the virus for the second time.
The country started its 42-day countdown towards being considered Ebola-free on March 4 and would have been clear by April 15 -- the much-touted "zero Ebola" deadline announced by the three countries.
The outlook has been mixed in Guinea and in Sierra Leone, which has announced a three-day lockdown at the end of the month of around 2.5 million people in a bid to stem the epidemic.
Sierra Leone counts the most cases of the virus at 11,794, but only 55 new confirmed infections were registered last week -- the lowest number since last June.