Military officials say 22 Marines have been treated for exposure to fire retardant gas at a Southern California desert base after an extinguishing system accidentally went off in an assault vehicle during a training exercise.

Base spokesman Dave Marks says an equipment malfunction caused the release of halon Thursday afternoon at the Marine Corps Air Ground Combat Center in Twentynine Palms. There was no fire or explosion.

The Marines were treated at the base hospital. Marks says three will be kept overnight for observation and the rest were returned to training.

He says the Marines were from a Hawaii base and were inside an amphibious assault vehicle that carries troops from ships to shore.

Twentynine Palms, 130 miles east of Los Angeles, is the largest Marine training base in the world.