A teenager has died in Liberia from the Ebola virus, the first such death since the country was officially declared free of the disease in September.
Francis Karteh, head of Liberia's Ebola crisis unit, said on Tuesday that the teenager's parents had also tested positive for the virus and were under observation in the capital Monrovia.
More than 150 people are under surveillance.
"The 15-year-old has finally died. He died yesterday," Karteh told the AFP news agency.
It was confirmed last week that the boy and two of his relatives had contracted Ebola, which has killed more than 11,300 people since December 2013 in its worst ever outbreak, mainly confined to the West African nations of Guinea, Liberia and Sierra Leone.
Confusion over age
WHO previously reported that the boy was 10 years old, but Karteh said he was in fact 15.
The boy fell sick on November 14 and was admitted to hospital three days later in Monrovia, the WHO said, adding that 150 people who had been in contact with the family were being monitored.
Liberia was first declared Ebola free in May, only to see the fever resurface six weeks later.
It was declared to have officially beaten the epidemic for a second time in September.
Carissa Guild, of Doctors Without Borders (MSF), said the fresh outbreak of the virus may reflect fatigue and complacency among health workers.
"There were no cases for a while and Liberia was nearing the end of a 90-day period of heightened surveillance ... It is quite possible that people were tired and got complacent," Guild said.
Since the beginning of the outbreak, Liberia has registered more than 10,600 cases and more than 4,800 deaths, according to a WHO report last week.
Karteh's death comes only days after Guinea's last known Ebola case, a three-week-old girl, was declared cured on November 16.
That announcement prompted a 42-day countdown - twice the incubation period of the virus - after which Guinea can be declared Ebola-free.
Sierra Leone was declared to have beaten the virus earlier this month.
The failure by the World Health Organization to sound the alarm until months into the outbreak was an "egregious failure" which added to the enormous suffering and death toll, a panel of global health experts said on Monday.