SAN ANTONIO -- An 18-wheeler being loaded with fuel at a San Antonio refinery exploded Wednesday, setting off a chain reaction of smaller explosions and sending a towering plume of thick black smoke over the city's southeast side.

San Antonio fire spokeswoman Deborah Foster said authorities were trying to account for all 100 employees at the AGE Refining Inc. facility and were evacuating residents within a one-mile radius. It was not immediately clear whether anyone had been killed or how many were injured. Foster said some workers were being treated at the scene.

She did not know the condition of the truck driver who was loading fuel at a tank late Wednesday morning when the blast occurred.

"What's bad is that he was at a fueling spot," said Foster, who noted that other trucks and drivers were in the area. "We've heard some minor explosions as we've stood here."

Foster said fire crews immediately 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 the fire could burn for some time.

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 were going through nearby apartment complexes with sirens, blasting an evacuation notice over the loud speaker and banging on doors and windows.

The Red Cross has 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.