PHOENIX - Seven workers at an Intel Corp microchip manufacturing plant outside Phoenix were injured by an explosion at the facility on Tuesday, and the building was immediately evacuated, fire officials said.

The blast of undetermined origin occurred shortly after 2 pm local time at one of two large Intel campuses in Chandler, Arizona, just southeast of the state capital, Chandler Fire Department Battalion Chief Brad Miller said.

He said that four workers were taken to local hospitals with injuries ranging from concussions to shrapnel wounds, and three others were treated at the scene for less serious injuries.

Intel, the world's largest chipmaker, was not immediately available for comment.

Miller called the incident a "flash explosion," adding that flames from the blast were quickly extinguished with a combination of water and foam. The extent of damage was not immediately clear.

Initial indications from the scene were that the explosion occurred inside a chemical storage room at the plant, which Miller described as a single-level facility where microchips are fabricated.

The plant and an adjacent multi-story building were evacuated of several hundred employees and will remain closed indefinitely as authorities investigate the cause, he said.

"Until we identify everything involved, there's no way we can say when people will be back to work," he told Reuters, adding that the 10 other buildings on the campus remained open.

Intel's website said its fabrication facilities in Arizona manufacture the company's Pentium processor family of products.

The company shares were holding steady in after-hours trading.