Liberia has confirmed a third new case of the deadly Ebola virus, as the disease has re-emerged in recent days nearly two months after the outbreak was declared over in the West African nation.

The discovery of new cases undermines the biggest success story of the West African Ebola outbreak which began last year, the deadliest ever.

Liberia, where 4,800 people have died out of the 11,200 killed in the outbreak, was declared Ebola-free on May 9 even as the disease has continued to infect new victims in neighbouring Sierra Leone and Guinea.

The first new case, 17-year-old Abraham Memaigar, died on Sunday in a rural area of Margibi County, about 30 miles from the capital Monrovia.

Deputy Health Minister Tolbert Nyenswah said the two other cases involved a 24-year-old and a 27-year-old who are both stable.

None of the new victims are known to have travelled to Guinea or Liberia, leading to speculation among the medical community that there could be hidden pockets of the disease or new means of transmission.