Tests on the Ebola virus that claimed Liberia's first victim since it was declared Ebola-free in May showed it was closely related to an earlier Liberian strain, the World Health Organization and a health official said on Friday.

The findings suggest the disease was never entirely eliminated from the West African country.

"It indicates that the virus is closely related to one that was circulating in Liberia in that particular area," WHO spokesman Tarik Jasarevic said, confirming initial results of the genetic tests obtained by Reuters.

"So it's either (from) a survivor or non-identified transmission in the community," he added.

Liberia's first Ebola case in nearly two months was reported on June 30 when the body of a 17-year-old boy, Abraham Memaigar, tested positive for the virus in Margibi County.

The case has baffled officials since Memaigar lived far from the hotspots of the epidemic on the borders with Guinea and Sierra Leone, and was not known to have been in contact with any travelers.

Ebola is thought to be able to survive no more than 21 days in most body fluids, such as blood and vomit, but is known to persist in semen and in some soft tissues, such as the eye, for months after recovery.