Forty people were reportedly injured — four critically — in a fire that ripped through a Brooklyn apartment building early yesterday.

The fire broke out in the four-story building on 61st Street near Fifth Avenue at about 5 a.m., officials said. It took 138 firefighters 2½ hours to get the three-alarm blaze under control.

The four people critically injured were residents of the building. Twelve firefighters were also hurt, one seriously.

Fire marshals were investigating the cause of the blaze.