Pittsburgh firefighters battled for about four hours against a stubborn fire at a Strip District warehouse since mid-morning.

The four-alarm fire at the structure at 25th Street and Smallman Street was reported just after 10:30. But old cork in the building, apparently used at one time for insulation, created a smoky situation and the fire is proved difficult to extinguish.

Smallman, meanwhile, remained closed between 24th and 26th streets until firefighters began wrapping up their gear and moving their apparatus.

The fire apparently started in an electrical conduit in a fourth floor ceiling in the five-story building, once a warehouse for the Otto Milk company. Workers renovating the building apparently sparked the fire.

A firefighter suffered a wrist injury.