A lithium battery fire inside a Franklin Township warehouse injured two firefighters early Friday morning.

Crews responded to the three-alarm fire on Belmont Drive around 4 a.m., where heavy smoke filled the building.

Somerset Fire and Rescue Chief Chris Gianotto says the fire broke out on the side of the warehouse used as a recycling center for batteries, computers and TVs. The other half of the building houses a furniture supply business.

Gianotto says lithium batteries made the fire difficult to extinguish because they can continue burning for an extended period. Crews were allowing the batteries to burn out while firefighters continued ventilation work.

Two firefighters were treated by EMS and have since returned to the firehouse.

"Everybody's OK. We had two firefighter injuries. They were treated by EMS and they are back at the firehouse," Gianotto said.

People inside the warehouse were able to get out on their own, and the sprinkler system activated.

Firefighters from surrounding towns were called in to help battle the blaze. Roadblocks and detours remain around the scene as crews continue their work.