AMHERST — A fire in an office on the third floor of a University of Massachusetts building Thursday morning caused extensive damage to the contents of the room and filled the hallway with smoke, according to Assistant Amherst Fire Chief Lindsay Stromgren.

Amherst firefighters responded to Arnold Hall at 5:57 a.m. when a fire alarm sounded in the building at 715 North Pleasant St. that houses offices for the School of Public Health and Health Sciences.

When a fire engine got there, smoke was found throughout the entire floor, prompting a box alarm to be sounded and bringing eight on-duty firefighters, along with off-duty firefighters, to the site, Stromgren said.

The fire was quickly extinguished, and firefighters remained on scene for about two hours, Stromgren said.

The fire was confined to a room, and while not much burned, the walls of the room and items inside it were blackened by the fire and are likely not salvageable, Stromgren said.

UMass spokesman Daniel Fitzgibbons said a mini-refrigerator that malfunctioned caused the fire, which resulted in smoke and soot throughout the third level. The third floor was closed Thursday, and the fourth floor was shut down until the air quality was monitored and deemed safe.

A contractor will clean up and the third floor could reopen Friday, Fitzgibbons said.

The building, a  former residence hall, has several offices, but no classrooms. No classes were affected, Fitzgibbons said. “It shouldn’t have had any impact,” he added.

No one was injured and only custodians were in the building at the time of the incident.

In addition to the firefighters, UMass Police and the UMass Office of Environmental Health and Safety responded. Amherst Police assisted with traffic control.

Scott Merzbach can be reached at smerzbach@gazettenet.com.

Scott Merzbach is a reporter covering local government and school news in Amherst and Hadley, as well as Hatfield, Leverett, Pelham and Shutesbury. He can be reached at smerzbach@gazettenet.com or 413-585-5253.