WINTHROP — Three children escaped unharmed from a Saturday morning fire that heavily damaged part of their family’s apartment at a four-unit building on Route 133, authorities said.

The fire, reported just before 8 a.m., appears to have started in the area of a Monitor heater and microwave in the kitchen, possibly as a result of combustible material being close to the heater, Winthrop Fire Chief Dan Brooks said.

The youngest child, Malachi Walo, 12, woke up breathing and smelling the smoke and alerted his older brother and sister about the fire, Brooks said. Walo, whose room was next to the kitchen, told responders that he noticed the smoke detectors were going off after the smell woke him, Brooks said.

“He did a great job,” Brooks said. “He basically discovered the fire and got everybody out of the house before there was a bigger issue.”

Brooks didn’t have the names or ages of rest of the family. The kids’ mother and her boyfriend weren’t home at the time but arrived shortly after the fire was reported, he said.

Winthrop, Monmouth, Manchester and Augusta fire departments responded to the fire. Crews knocked down the fire in about 20 minutes and cleared the scene by 10 a.m., Brooks said. Two of the other three apartment units were occupied, but the damage was contained to the one unit, he said.

No one was harmed in the fire. The family in the damaged apartment unit will need to find somewhere else to stay, Brooks said. The American Red Cross is assisting the family through the weekend, he said.

The residents in the other two occupied units were expected be able to return to their apartments by Saturday night, Brooks said.

The apartment building is owned by Patrick Rand, according to Brooks. The building was insured, but the family in the damaged apartment didn’t have renters insurance for their belongings, he said.

Brooks said the family will be able to salvage most of their belongings, but extensive repairs are needed before they can move back.