O Globo identified at least six of the dead as youth players, but the only one named by a club official was Christian Esmerio, a 15-year-old goalkeeper. As much as we tried to get inside and find people still alive that wasn't (possible).
Local media reported that the fire happened in a dorm where youth players sleep.
The cause is not yet known.
"This is certainly the worst tragedy that this club has gone through in our 123 years with the loss of these 10 people".
Outside the complex, an AP reporter saw two ambulances and a fire truck enter.
The club posted a graphic on Twitter that said "Flamengo is in mourning".
The Rio de Janeiro mayor's office said the dormitory destroyed by the blaze was built on land that was not registered for buildings and that the club had no permission to erect lodgings there. "There is no register of a new request for a license to use the area as a dormitory". The fire department have said that they can not yet discount the possibillty that they will find more bodies.
Joao Pedro da Cruz, a 16-year-old player in the Flamengo youth league, told G1 news portal that he decided not to stay the night at the facility Thursday because the team wasn't going to train on Friday.More news: Drug Bust: Police Warn Mexican Cartels Targeting Australia
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Most of the youth players at the club are not locals but come from all over the country.
"For poor kids, many see it as the only path", he said.
Some of its most prominent former players include World Cup winners Ronaldinho, Bebeto and Romario, and more recently wonderkid Vincius Jr.
After years of financial difficulties, Flamengo last year spent 23 million reais ($6.2 million) to expand the Ninho do Urubu training ground, which was being used both by the first team and youth squads.
The centre is located in the west of Rio de Janeiro, and according to the city's fire department ten lives have been lost.
"We are extremely sad and shaken by the news of the fire", tweeted Chapecoense, a team in southern Brazil that lost 22 players in a plane crash in 2016. "May the Black and red nation have the strength and faith to overcome this moment".
Messages to Flamengo officials were not immediately answered.