A woman suffered serious injuries after she jumped from a window three floors above the ground to escape a fire in her Fitchburg apartment building Tuesday morning, ­officials said. Five others, includ­ing a firefighter, were ­also injured in the blaze.

Fitchburg Fire Chief Kevin Roy said the woman, whose name was not released, was hanging from a bedroom window while firefighters struggled to reach her with a ladder, impeded by burning electrical wires leading to the house.

“She was in bad danger from heat and smoke,” he said. “One of the lieutenants ­instructed her to jump toward the tree, and she did.”

She was taken to a local hospital and is expected to survive, Roy said.

Officials believe the three-alarm fire began at about 8:30 a.m. on the third floor of the four-story apartment building at 124 Daniels St.

‘One of the lieutenants instructed her to jump toward the tree, and she did.’

The building is home to about two dozen people, Roy said.

Roy said Fire Captain Dave Rousseau, who was off duty at the time, saw the flames and ran into the building. He helped a woman and a sleeping man escape from upper floors.

Mayor Lisa Wong said that Rousseau was assisted by ­Robert Eyels, who owns Eyels Electric.

Firefighters were alerted to the emergency by two women who saw the fire and walked about four blocks to the station to report it, Roy said.

By the time firefighters ­arrived, flames were shooting out of the third- and fourth-floor windows.

The injured firefighter, one of the first on the scene, suffered burns to his head when flaming debris fell on him, Roy said. He was treated and ­released from a local hospital.

At one point, firefighters were ordered to evacuate the building and fight the blaze from the exterior.

The apartment building next door also caught fire, Roy said. It melted the siding but did not reach the interior. Residents will be able to return to that building once power is ­restored, he said.

Roy said the fire was brought under control in about 40 minutes, but it still took several hours to completely extinguish.

Several residents were transported to local hospitals, mostly suffering from stress-related issues.

St. Joseph Church, located across the street from the fire, was used as a temporary shelter for a handful of residents.

Janice Potter,an office manager at the parish, said that when the fire began she went out and saw “flames shooting out of the top.” She said she could smell the smoke from within the church, located in the city’s Cleghorn neighborhood.

“We used the church as a service saving ground,” Mayor Wong said.

“Cleghorn is a very tight neighborhood and something like this has an effect on the entire neighborhood,” she said. “One woman showed up with the right-sized diapers, so I think that indicates how well people know each other in the neighborhood.’’

According to the city assessor’s office, the wood-framed building is worth about $145,000 and was built in the early 1900s. It contains six units.

Roy said it would be up to the owner of the building to decide whether it will be ­repaired.

He could not estimate the damage amount but said the third and fourth floors sustained extensive smoke and fire damage, while the first and second floors had a lot of water damage.

The Fitchburg arson squad and state fire marshal’s office are investigating the cause of the fire.

Roy said it appears the flames started on the third floor.