A brush fire has erupted off Interstate 5 near Castaic in the Angeles National Forest and is burning through heavy brush and slowing traffic.

Los Angeles County Fire Inspector Steve Zermeno says the blaze was reported at 4:47 p.m. Wednesday and has burned 20 to 25 acres.

Zermeno says no structures are threatened. He says helicopter and fire crews are fighting the flames and making good progress.

The California Highway Patrol says all lanes of northbound Interstate 5 were closed for more than an hour in the Castaic area because of smoke from the fire.

Zermeno says a witness told CHP that they saw two men with long hair in a green Saturn at the scene and they might have started the fires.

Castaic is about 40 miles northwest of Los Angeles.