"Over 680 people in the village of Robureh are now under a 21-day quarantine," Amadu Thullah, a spokesman for the local Ebola response centre told AFP.
The centre said those locked down included her parents, close relatives and classmates.
"They are classified as high risk although they have not exhibited any signs and symptoms of the disease," added health ministry spokesman Seray Turay.
"The surveillance team of the Ebola response centre have intensified their investigations and is working to nip the issue in the bud."
The girl`s death came two weeks after a 67-year-old food trader was killed by the tropical fever in the neighbouring district of Kambia, but the two outbreaks are not linked.
The National Ebola Response Centre (NERC) said 1,524 people were in quarantine across the two districts.
A spokesman for the local response team told AFP morale was extremely low in affected area of Makeni, the country`s largest northern city.
"It is a wake-up call that Ebola is still in the country but we have an overwhelming turnout of our partners (and) the coordination response is fantastic," he added.
On August 24, President Ernest Bai Koroma led a festive ceremony celebrating the discharge of Sierra Leone`s last known Ebola patient, from a Makeni hospital.
No new cases had been recorded in more than two weeks, allowing Sierra Leone to join neighbouring Liberia in the countdown to being declared Ebola-free.
The city is located in Bombali district, bordering Guinea. The district last reported a case nearly six months ago, official records show.
Robureh residents told AFP by telephone the "once vibrant community" had become "as silent as a graveyard" with people shut in their homes.
"Not even the chirpy songs of birds are heard," said 45-year-old farmer Alimamy Sesay, whose sister is in quarantine.
"Everything is at a standstill, few people woke up this morning to go to their farms and I dont know what tomorrow will bring."
Officials figures show the west African outbreak of Ebola has killed more than 11,000 of 28,000 people infected since first emerging in December 2013 in Guinea, with Liberia the hardest hit.
Experts acknowledge that poor monitoring, especially early in the outbreak, means that the real death toll could be signficantly higher.
More about:
