Liberia requested US assistance after three new cases were reported on Friday, said Dr. Francis Kateh, the country's chief medical officer and acting head of Liberia's Ebola Case Management System.
The first of the latest patients is a boy from the capital, Monrovia, who officials initially said was 10 but now say is 15. The boy's father and brother also have tested positive for Ebola and have been taken to the same treatment center along with his mother and two siblings.
Liberia has recorded more than 4,800 deaths during the worst Ebola outbreak in history, according to the World Health Organisation.
More than 11,300 deaths have been recorded for the entire outbreak, which was concentrated in Liberia, Sierra Leone and Guinea, according to WHO. The outbreak was confirmed in March 2014.
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Liberia was first declared Ebola-free on May 9, but new cases emerged in June resulting in two deaths. WHO declared the country Ebola-free again on September 3.
Guinea had been the last country with any cases after Sierra Leone was declared Ebola-free on November 7.