The authorities in Omaha urged some residents to evacuate on Monday after a fire at a chemical plant sent clouds of black smoke billowing over the city, officials said.

The fire started just before 7 p.m. at a plant owned by Nox-Crete, a company based in Nebraska that manufactures chemical products, Scott Fitzpatrick, a battalion chief with the Omaha Fire Department, said.

There were no immediate reports of injuries. However, the authorities urged people who live near the Nox-Crete plant, which is south of downtown Omaha, to evacuate Monday night because smoke from the fire was threatening to blow over a residential area, Chief Fitzpatrick said.

“We’re unsure what exactly is in there, but we do know there are some chemicals that are burning,” he said, adding that about 50 firefighters were still battling the blaze late Monday night.

Nox-Crete did not immediately respond to requests for comment.