PHILADELPHIA >> Explosions and a blaze at the largest oil refinery on the East Coast sent a fireball into the sky and shook homes before dawn today, 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 this 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.

The 150-year-old oil refining complex processes 335,000 barrels of crude oil daily, PES says. The refinery turns the crude into gasoline, jet fuel, propane, home heating oil and other products.

It was the second blaze at the refinery this month, following a June 10 fire in which no injuries were reported.

Gasoline prices in the Northeast could rise over the next few weeks as a result of the explosion, but an increase isn’t likely to last long, said Claudio Galimberti, a refining analyst at S&P Global Platts.

Thick black smoke billowed across the city after the fire started.

Jonathan Triboletti, of Hammonton, New Jersey, saw that smoke and the explosion when he and his mother were driving to the nearby Philadelphia Airport.

“I noticed the sky light up orange ahead of us for maybe 5 or 6 seconds,” he said.

Triboletti said he even felt the heat in the car.

“It kind of made our skin crawl a little bit because we didn’t know what was going on. It was something you normally see in a movie theater,” he said.

It’s unknown whether the explosions will cause any financial strain the oil markets.

“This is obviously not a good thing to happen in a jittery market, but U.S. refined product demand has been relatively weak,” said Jim Burkhard, vice president for oil markets at IHS Markit. “So this is a disruption, but it’s probably not going to cause major problems in a market that is generally well supplied.”