MONROVIA (Reuters) - A 15-year-old boy has died of Ebola in Liberia, the first such fatality for months in a country declared free of the disease in September, its chief medical officer said on Tuesday.

The boy tested positive last week and died late on Monday in hospital in Paynesville near the capital, where his father and brother are also being treated for Ebola, officials said.

Liberia has placed under surveillance 153 people who may have come into contact with the boy. A further 25 healthcare workers are being monitored, of whom 10 are identified as high-risk, chief medical officer Francis Kateh said.

The source of the virus is being investigated and Liberia has requested the assistance of two experts from the Centers for Disease Control and Prevention in the United States.

"Our working hypothesis is that the virus is reintroduced into the human population through uninfected people and we know that it is a possibility that people who have been infected with the virus previously may continue to transmit," said the Liberia country representative for the U.N. World Health Organization, Dr. Alex Gasasira.

The U.N. health agency has twice declared the West African nation Ebola-free, once on May 9 and again on Sept. 3. Liberia's last Ebola death was in July.

More than 11,300 people have died of the virus since it broke out in West Africa in March 2014. Of the three main countries affected, Sierra Leone was declared Ebola-free on Nov. 7 and Guinea started its countdown to zero on Nov. 16. Over 4,800 people died of Ebola in Liberia, according to WHO figures.

Liberia is working with international partners including the WHO on a protocol so that a trial Ebola vaccine can be administered to people who might have been exposed to the new confirmed cases, Gasasira said.

Trials of the vaccine have proven effective in Guinea and Sierra Leone, he said.

(Writing by Makini Brice; Editing by Matthew Mpoke Bigg and John Stonestreet)