"It's a very dangerous situation," Hood said, who noted that they were still trying to determine what kind of fuels are burning. "We will be out here quite a while."
Several employees were treated at the scene and everyone except the driver of the truck that exploded was accounted for, Hood said.
Residents within a half-mile radius of the refinery were evacuated and Hood was urging those with respiratory illnesses to remain inside as smoke rose in a large black column from the site.
San Antonio fire spokeswoman Deborah Foster said fire crews sprayed foam to tamp down the flames because water is useless against a petroleum fire. They were bringing more foam in as fast as they could, but that the fire could burn for some time if crews couldn't reach the shut-off valves.
More than 100 firefighters were on the scene, and Hood said commanders would have to move to avoid smoke as the weather changes later in the day.
Foster said that allowing the fire to burn out on its own would be a difficult call because the smoke is black and acrid and there are a cluster of apartment buildings nearby.
Vanessa Valdez, 23, said she heard something like "gunfire" from her apartment about a mile from the refinery before a swarm of firetrucks and ambulances raced past. She was later evacuated from her home.
Police went through nearby apartment complexes with sirens, blasting an evacuation notice over the loud speaker and banging on doors and windows.
The Red Cross set up a shelter at a local school for evacuees.
AGE marketing director Jeff Dorrow confirmed that a truck exploded at the refinery that handles about 14,000 barrels per day, but he had no other immediate details.
AGE, which runs only the San Antonio facility, is a small refiner and is currently operating under bankruptcy protection.
