Liberia has announced the return of the deadly Ebola virus, more than six weeks after the country appeared to have eradicated the disease.

"A new case of Ebola has been reported in Margibi County," said deputy health minister Tolbert Nyensuah on Tuesday. "The person has died and was confirmed positive before death. He has been buried."

The official told a radio station 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.

Liberia's neighbours Guinea and Sierra Leone are both still battling the outbreak, which has killed more than 11,000 people, but the coastal Margibi County is much nearer the capital Monrovia than either border.

The epidemic killed more than 4,800 Liberians before the World Health Organisation declared the country Ebola-free on May 9, 42 days after the last confirmed case was buried.

WHO spokesman Tarik Jasarevic told reporters in Geneva the UN health body had been informed of the case.