MIAMI (AP) _ Two small explosions rocked a burning office building early today, injuring nine firefighters including two who were trying to rescue a trapped colleague at the time, authorities said.

None of the injuries was serious. Most were cuts and bruises.

``We're very lucky,'' Fire Chief Carlos Gimenez said. ``If any of these projectiles we saw inside struck a firefighter, you'd have much more serious injury.''

The three-alarm fire heated up a drum of hydraulic oil in an elevator mechanism closet and spread gas vapors throughout the building before exploding, Gimenez said.

The force of the blast blew out doors, windows and blinds on all four sides of the two-story concrete-block office building on Biscayne Boulevard about four miles north of downtown Miami. A piece of aluminum shot through one door.

The trapped firefighter was freed after the explosion. It was unclear why he had been trapped.

The three firefighters caught inside and four outside were taken to the hospital. The most serious injury was a 4-inch gash on a firefighter's forehead, authorities said.

Two other firefighters injured outside were treated at the scene.