"We can't tell you the cause of the fire, that needs to be investigated", he added.
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.
The ages and identities of those killed were not released but the three injured were 14, 15 and 16 years old, the fire official said.
He did not take questions from reporters who also were outside the complex but added: 'The most important thing right now is to minimise the suffering of these families'.
"The air conditioner caught fire, and I ran out", Felipe Cardoso said, according to BBC News. Images displayed by Globo TV from a helicopter showed smoke emerging from a charred area.
While there is still no information on what caused the blaze, it was brought under control within two hours.
Real Madrid star Vinicius Jr came through Flamengo's system and not long ago was living in the training facility.
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Family, friends and fans gathered to pay their respects at a mass ceremony held in memory of the young players and staff lost in the Flamengo football club fire..
This is certainly the biggest tragedy this club has ever experienced in its 123-year history with the lives of these 10 people lost.
"I am very happy". Instead, he went to a friend's house. 'Today I woke up and heard this awful news'.
Of the ten, four are Flamengo youth team players, two are young boys on trial, and four are club employees.
The dream of many youths in Latin America's largest nation, victor of five World Cup titles, is to make it into the ranks of professional soccer.
Flamengo have a youth development programme for promising young players in their early teens.
Like many clubs who sign youngsters from across the country, the facility has accommodation for teenagers, although it was unclear exactly how many players were among the dead.