"A new case of Ebola has been reported in Margibi County. The person has died and was confirmed positive before death. He has been buried," said deputy health minister Tolbert Nyensuah.
The official told a radio station the experts had traced and quarantined anyone who may have had contact with the victim, without giving numbers or any details on the patient.
"We are investigating to know the origin of this new case. We ask all Liberians and all other nationals living in Liberia to continue taking the preventive measures," he said.
World Health Organisation spokesman Tarik Jasarevic told reporters in Geneva the UN health body had been informed of the case.
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The epidemic killed more than 4,800 Liberians before the WHO declared the country Ebola-free on May 9, 42 days after the last confirmed case was buried.
That period is double the number of days the virus requires to incubate, and WHO hailed its eradication as an enormous development in the long crisis.
"We must not let down our guard until the entire region reaches and stays at zero Ebola cases," he said on May 9.
The WHO also warned that because the Ebola outbreaks were continuing in neighbouring Guinea and Sierra Leone, the risk remained high that infected people could re-enter the country.
Because of that risk, medical aid agency Doctors Without Borders, at the forefront of battling the outbreak since it emerged in Guinea in December 2013, was also muted in its applause of the WHO's assertion that Liberia had beaten Ebola.
Sierra Leone and Guinea had been seeing numbers of new cases drop dramatically but the decline has halted and the countries have been recording around 20 to 27 new cases a week since the beginning of May.