A woman sustained severe burns in a four-alarm fire at an apartment building in the North End late Tuesday night, in a blaze that brought about 100 firefighters to the scene.

Fire Department spokesman Steve MacDonald said the fire at 26 Cooper St. started shortly after 11 p.m., and that the unidentified woman was taken to Massachusetts General Hospital with severe injuries. No other injuries were reported.

The blaze required the large number of fire personnel because of concerns that the fire could spread to other buildings, MacDonald said. But, he said, firefighters were able to contain the blaze to the three-story brick building.

The cause of the fire was under investigation. All residents were also able to escape the building, but fire officials did not immediately know how many people lived in the apartments. The city assessing database lists the building as a four- to six-family dwelling.

“It’s hard in the North End, because buildings are so close, and it can be very dangerous, because fire can spread,” said Pasqua Scibelli, 40, who was evacuated from an adjacent apartment building, along with her 9-year-old son.

Dozens of people crowded outside early Wednesday morning as smoke escaped from the roof and the windows.

Michael Allen, 23, of Boston, who was visiting a friend in the building next door, walked outside for a late-night snack and saw the fire engulf the building’s top floors within minutes.

Allen said he and another passerby helped two young women jump out of a first-floor window.

Katelyn Whitcher, 27, who lives in the neighborhood, said she saw the flames as they shot through the roof.

“I saw the windows break and flames gushing out of the top floor. It was pretty big, like something out of one of those . . . movies,” she said.

MacDonald said the fire did go through the roof, but he was unsure whether the roof had collapsed.