An official in the West Africa country that has been devastated by the worst ever outbreak of the virus urged calm, insisting that health workers had the means to contain the latest outbreak.
"We are calling on the population not to panic because we have people capable of putting the situation under control," health ministry spokesman Sorbor George told reporters.
"Let everyone take the necessary measures to prevent the virus from spreading," he added.
The other two patients are related to the boy.
Also Read
WHO's Ebola response chief, Bruce Aylward, told journalists in Geneva that initial investigations have not yet turned up a confirmed link between the 10-year-old and another Ebola sufferer, leaving many questions unanswered as health officials try to figure out the origins of the new case.
"The child has no known history of contact with a survivor or having been at a funeral" where an Ebola victim was buried, Aylward added, a reference to the fact that secretions from the corpses of those who died from the virus are highly contagious.
Aylward said that two of the boy's siblings had reported feeling ill over the last two days, but there was no confirmation that they were the new confirmed cases.
"His father and mother had transient illnesses in the last few weeks, so obviously that could be one of the lines of transmission," Aylward also said.
Liberia, where thousands died at the height of the epidemic last year, was first declared Ebola free in May, only to see the fever resurface six weeks later, and again in September.