BOERUM HILL - Four children and their grandmother are recovering after a fire ripped through their Boerum Hill apartment early Sunday morning.

The FDNY says the fire started at around 2:15 a.m. on the fifth floor of the Wyckoff Gardens Houses on Third Avenue. Rookie firefighter Jordan Sullivan says a woman told him her grandchildren, including a 5-month-old baby boy, were stuck inside the apartment.

Sullivan says the smoke was so thick he could barely see, but he managed to rescue the baby, along with three other children.

The baby and his 8-year-old sister are both listed in critical, but stable, condition. The two other children, ages 14 and 8, and their grandmother are being treated for minor injuries and are expected to be OK.

The cause of the fire remains under investigation.