Fire crews battled a large blaze at a meatpacking plant in a crowded suburb south of Milwaukee on Monday, and authorities warned residents within a mile of the plant to leave the area because of the threat of toxic fumes.

The fire started Sunday night in an area between the sprinkler system and the roof, Fire Chief Dan Mayer said. About 100 firefighters were still battling the blaze Monday morning at the Patrick Cudahy Inc. plant in Cudahy, seven miles south of downtown Milwaukee. No injuries were reported.

Nearby residents were told to evacuate the area because of possible ammonia fumes, Mayor Ryan McCue said.

Most of the ammonia at the plant is contained, but if flames breach a fire wall it's possible the gas could be released in an explosion, authorities said.

Ammonia can be extremely toxic and may be fatal if inhaled. The federal Emergency Respond Guidebook says ammonia vapors are irritating and corrosive and exposure can cause respiratory difficulties.

A local school was made available for evacuees and their pets to take shelter. Summer school activities were also canceled Monday.

Rebecca Spencer, who lives about a quarter mile from the plant, left her home with her dog and two cats. She said she wasn't worried when she saw huge plumes of black smoke because she was confident firefighters would contain the blaze.

Carla Peterson, a plant official, said only a few of the company's roughly 2,000 workers were at the 128-year-old plant when the fire started, and that no one was hurt.

Mayer said the fire was difficult to fight because it is in a confined space. He said there was heavy fire, water and smoke damage to the plant.