The victims were in two apartments on the third floor, State Fire Marshal Stephen Coan said. He said some residents have not been accounted for.
Witnesses said some people jumped from windows and threw children out in an effort to escape the flames. Rotha Proeung told the Boston Herald his son and daughter were trapped in an apartment with their mother and her boyfriend, but the adults managed to get the children to a window where firefighters reached them.
"They were very frightened," Proeung said of his children, who were treated for smoke inhalation.
Officials said 48 people were believed to be living in the nine apartments in the building, which had shops on the ground floor.