Over 100 firefighters are battling an explosive fire in downtown Los Angeles Tuesday morning.

The fire broke out at a three-story building in the 200 block of S. Los Angeles Street, near 3rd Street shortly before 2 a.m.

Los Angeles Fire Department says it has declared it a “major emergency” fire, and 150 firefighters are “still in an active firefight.” No injuries have been reported.

Small popping sounds could be heard in video from the scene, and flames were seen on both sides of the street. A homeless encampment across the street caught on fire, LAFD said.

The Red Cross is assisting those that were displaced by the fire, LAFD Public Information Officer Erik Scott said.

There are nearby residential buildings but it was not immediately clear if residents were asked to evacuate.

Photos: Firefighters Battle Explosive Building Fire in Downtown LA

At least four businesses are being impacted by the fire, including a smoke shop, said Los Angeles Fire Department Spokesman David Ortiz.

Scott added that a significant explosion occurred and rolling steel doors were blown out onto the street.

Nitrous oxide canisters were found at the scene, Ortiz said. LAFD arson investigators are at the scene.

The three-story building impacted by the fire is about 100 years old with weaker walls and an unreinforced structure, according to Ortiz who added that an Urban Search and Rescue team is at the scene monitoring any shifts in the building due to concerns of a collapse.

The immediate area is shut down and will remain so for several hours, LAFD said. Bus service and commuter traffic will be impacted in the area. Residents are being asked to stay away.

The cause of the fire is under investigation.

This is a developing story. Please check back for updates.