Explosions and a blaze at the largest oil refinery on the East Coast sent a fireball into the sky and shook homes before dawn Friday, though authorities reported only a few minor injuries and said the air was safe to breathe.

The fire at the Philadelphia Energy Solutions Refining Complex was deemed contained and controlled but continued to burn late Friday afternoon, said Craig Murphy, deputy fire commissioner.

The fire broke out around 4 a.m., PES spokeswoman Cherice Corley said. Video showed the enormous orange blast bursting into the sky about 20 minutes later.

Five refinery workers were treated for minor injuries, and nearby residents were asked to stay inside.

The cause of the fire was still unclear. Three explosions, felt miles away, went off as the fire worked its way through the tangle of pipes carrying fuel across the complex, Corley said. It happened at the Girard Point refinery, one of two at the PES complex in southern Philadelphia.

The fire happened at a tank containing a mix of butane and propane, Murphy said. The blaze was still being fed by a main, he said, but it was unsafe for workers to access the valve to shut it off. If firefighters extinguished the blaze, the gases would continue to bleed into the atmosphere, he said.

"It is safer if it burns itself out; otherwise the product would be looking for an ignition source," he said. "Right now, it is contained and controlled."

Air samples taken both downwind and upwind from the refinery have been tested, and no threat to public safety has been found, the city's health department said in a statement.

(With input from AP)